Fix proguard notes about unkept descriptor classes

Closes #688

See merge request !366

Fix crash when navigating back in contact selector

Closes #718

See merge request !365

Disable EmojiTextView software layer rendering when cache is too small

This needs to be tested if it works as intended on several devices.

See merge request !362

Limit scene transition animations to API 23 and above

to work-around [android bug #224270](https://code.google.com/p/android/issues/detail?id=224270).
This is only necessary if the transitioning view might not be available anymore when the exit transition is made.

Closes #686

See merge request !361

Implement UX for creating a private group

This MR allows the user to create a new private group and select contacts to be invited into the group.

There are currently 6 commits starting with some small refactoring for code reuse and making more functionality available in the backend. Each commit could be split up into a dedicated MR if desired.

![create-groups](/uploads/5229f102ee2611c05d5e9d1e2aac510d/create-groups.gif)

Closes #661

See merge request !353

Truncate all messages to valid length before sending



See merge request !358

to work-around android bug #224270.
This is only necessary if the transitioning view might not be available
anymore when the exit transition is made.

Fix race conditions when updating UI from events (again)

This is my second attempt at fixing race conditions caused by updating the UI from events while background tasks are loading data from the DB. Unlike my first attempt, this one is pretty simple and doesn't require too much reasoning about possible races.

The first commit fixes a few list loading bugs I found while working on this problem, and moves the lifecycle callbacks from resume/pause to start/stop, closing #609. The second commit contains the fix for #705, which works as follows:

* Each BriarAdapter has a revision counter
* Before making a change to the adapter that could be overwritten by a background task, increment the revision
* Before starting a background task that could overwrite other changes, get the current revision
* Before applying changes from a background task that could overwrite other changes, check whether the revision has changed
* If the revision has changed, restart the background task
* Otherwise apply the changes

Closes #609. #705 remains open because the PagerAdapters for blogs need to be updated.

See merge request !356

All changes to the adapter are cumulative.

Also fixed a couple of bugs.

Throw FormatException if BdfList index is out of bounds

Closes #712

See merge request !359

Private Group Threaded Conversation

This MR refactors the forum activity, its controller, its adapter and view holder so *most* of the code can be re-used for private groups by making heavy use of generics.

The refactoring has 1383 additions and 1087 deletions, so just grows the code-base slightly and adding the private group conversation just takes an additional 400 lines.

The MR also includes one commit that moves post/message creation more into clients, so the UI doesn't need to keep track of timestamps. This commit can of course be split out into a separate MR if desired.

Closes #662, #663 

See merge request !350

This way the forum and private group client do not need to keep track of
message timestamps themselves and do not need to interact with
post/message factories.