Fixes the following crash that occurs after upgrading to support 23.3.0:
https://code.google.com/p/android/issues/detail?id=174871

The new activity shows who you are sharing a forum with and who shares a
forum with you. It is accessible from the overflow menu when in a forum.

Closes #398

Closes #399

Closes #396

Closes #395

Closes #374

Also prevents the snackbar from hiding the bottom of the list.

Closes #337

This changes `ShareForumActivity` to use two fragments to facilitate
forum sharing with the new Forum Sharing Client backend.

The `ContactSelectorFragment` allows the user to select a
number of contacts. If there is an ongoing sharing session or the forum
is already shared with the contact, it is disabled in the list. If there
is at least one contact selected, a button appears in the toolbar that
brings the user to the `ShareForumMessageFragment` where the user can
write an optional message to be send along with the invitation.

After sending an invitation, the user is brought back to the forum that
she shared and there is a snackbar showing up briefly to indicate the
successful invitation.

The invitation is shown along with the message within the private
conversation of each contact. The person who shares the forum also sees
the invitation and the message as outgoing messages that also display
the current status of the messages.

A notification is shown like for other private messages as well.

Please note that this commit does not include a way for users to respond
to invitations.

Turn list of available forums into a BriarRecyclerView with XML layout.
Allow to respond to forum invitations from the list of available forums.
The user can either accept or decline an invitation.

Includes parallel improvements to Bluetooth key exchange UI.

The adapter of the `ForumListFragment` has been changed into a
`BriarRecyclerView` and all its code has been adapted and simplified
accordingly.

All UI of the forum list is now defined in XML layouts.

part of UI changes for #305

Methods for creating, adding and removing forums have been moved to the
`ForumManager`. In order to still handle removing forums properly, a
`RemoveForumHook` has been introduced.

Methods for sharing forums with all current and future contacts have
been removed along with the localGroup where this information was saved.

The `ShareForumActivity` now has the proper label.

The `SessionId` and the `ProtocolEngine` have been moved to the
`clients` package.

This addresses part of #322 and part of what has been discussed in #320.

This commit is a small change that replaces the custom UI elements in
the forum list with a Snackbar and an Toolbar menu.

Addresses one part (2b) of #305

Closes #307

This commit introduces an abstract `BaseContactListAdapter` which provides
most of the adapter logic. The original `ContactListAdapter` extends it to
show date and online status of the contacts.

The new `ContactChooserAdapter` which is used for introductions extends
the `ContactListAdapter` and adds logic for graying out contacts from
different identities than the currently used one.

A new `ContactSelectorAdapter` extends the `BaseContactListAdapter` and
allows to select multiple contacts. It offers a method to return a
collection of all selected `ContactId`s.

This commit also sneaks in an animation when the 'Share Forum' button
is clicked.

Closes #292

Removes all programatic calls that modify the layout and uses the XML
resource instead.

This also introduces constants for the GROUP_ID, FORUM_NAME and
MIN_TIMESTAMP as they were used for the two buttons.

Closes #313

* If the user has already declined, we don't show that the other
  introducee has declined as well. The backend doesn't have that information, so
  this is compatible with the principle of showing what we know.
* If the user has already accepted or hasn't yet responded, we show the
  decline response in the private conversation with the introducer. If
  the user hasn't yet responded, we hide the accept/decline buttons
  in the introduction request message.

Messages an introducee receives in a `FINISHED` state are now being
ignored and deleted.

Closes #295

Will currently fail at runtime; requires a public key and a server onion.

Closes #293

* force decline when two of our own identities are introduced to each
  other
* throw away introduction requests to the same identity
  (impossible to trigger from UI)

Closes #284

(except refactoring of conversation item classes)

When devices' clocks are out of sync, it is possible that a response is
shown before the request. This commit makes sure that the timestamp of
responses is always later than the last message in the conversation.

Some wording could be misunderstood to thing introductions were
successful even though they were not. That has been clarified.

A new database transaction was created when getting contacts and local
transport properties. This has been changed to re-use the existing
transaction.

Also addresses minor issues found in review.

Show system notification for successful introductions