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

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.

Merge branch '670-uncaught-exceptions-do-not-print-a-stack-trace-in-introductionintegrationtest' into 'master'

Print stack trace for uncaught exceptions during tests for easier debugging

Closes #670

See merge request !355

Use constructor injection for controllers

Also made some listeners volatile.

This is part of #704 - if I don't find any other classes that need constructor injection I'll close the ticket.

See merge request !351

Null safety annotations

The @NotNullByDefault annotation marks all fields, methods and parameters in a class or package @NotNull, so Android Studio will warn if values that may be null are used. Please use this annotation for new classes, and specify @Nullable for any fields, methods and parameters that may be null.

Injected fields are initialised to null, so injected classes should use @MethodsNotNullByDefault and @ParametersNotNullByDefault, or specify @Nullable for injected fields.

See merge request !349