Add functionality to save and restore recently used Emojis

Update emoji and add new categories based on AOSP's XML file

Also replace more raster graphics with vector graphics.

Closes #311

Fixes #539, #564 and #568.

Also properly handle back navigation when trying to add contacts.

Closes #342

Only personal blogs from non-contacts can be removed.
This also adds integration tests that check if blogs can actually be removed.

Closes #579

This refactors the forum invitation code,
so it can be used by both: forums and blogs.

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.

Closes #517

This also adds a setting allowing people to turn blog post notifications off.

Closes #488

Closes #472

Closes #415

Closes #533

* Explain what "account creation" means on first run
* Use hints instead of dedicated text views
* Move password strength meter up into the user's view
* Always move user's view to current input field
* Improve "Forgot Password" dialog

Closes #151

Closes #360

Closes #455

This commit addes a combined blog feed that shows all posts of all
subscribed blogs in the order the blog posts have been received.

For now, this commit also hides other blog functionality like adding
additional blogs and browsing individual blogs.

Closes #417

Closes #418

This allows for swiping left/right to read other posts by using
a ViewPager.

This hasn't been done as a separate activity, but with
fragments, so both can share the `BlogPersistentData` without
needing to reload it.

Closes #428

Closes #411

This commit lays the groundwork for #415

This does not yet support multiple identities. It just shows blogs
created by the first identity, but can easily be adapted for
multi-identity support.

Closes #410