All text that can be generated by users will show emoji from the
shipped sprites.

For all messages and posts, the custom emoji keyboard is now available.

This also disables menu actions rather than hiding them and thus
closes #677

Included is a fix for a regression that was not showing the keyboard
automatically in forums and thus
closes #676

This commit refactors the code for sharing forums,
so it can be used for sharing blogs as well.

It does not yet include code for responding to blog invitations.

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.