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

Closes #337

Switched AppBus for ResultHandler, Controller for Helper. Added the basics for LifecycleControllers and implemented it for BriarActivity and NavDrawerActivity

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.