Forum Sharing Client UI

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. This is in MR !172 (merged). Both MRs are based on the new backend in !170 (merged).