Add first onboarding screen

When the user enters a private conversation after adding her second
contact, an onboarding screen will be shown highlighting the possibility
of introducing the contacts to each other.

![intro2](/uploads/b2cd4377f39974ca1fbd95649558e487/intro2.gif)

Closes #357

See merge request !324

When the user enters a private conversation after adding her second
contact, an onboarding screen will be shown highlighting the possibility
of introducing the contacts to each other.

Emoji

This MR introduces a custom Emoji implementation to Briar for devices that do not support Emoji sufficiently. It is heavily based on Signal's implementation. Hence, the license for the Android part has been changed to GPLv3.

So far, emoji input is only supported for forums and private conversations that both rely on the same `TextInputView`.

![emoji2](/uploads/f69af031e72efb0bd6f0f67a9574d11f/emoji2.gif)

See merge request !317

Add functionality to save and restore recently used Emojis

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

Don't offer messages back to the sender

Closes #538

See merge request !323

Avoid repeated author status lookups

Closes #625

See merge request !322

Include stream number in stream header nonce

See the corresponding change in the BTP spec for an explanation:
https://code.briarproject.org/akwizgran/briar-spec/commit/388e1d23c04c2f6954d13d36ffc231611a3c6fa9

Closes #329

See merge request !320

Fix potential NPE in AndroidExecutorImpl

Always wait for the startup latch to ensure the background thread's Handler is initialised before using it.

Closes #665.

See merge request !319

Don't throw an exception if the client has no delivery hook

Closes #664.

See merge request !318

Clean HTML from RSS feeds with Jsoup and Show Link Warning

This adds HTML cleaning with the Jsoup library based on a whitelist. The resulting HTML is then used to create a `Spannable` in Android. This spannable is traversed and URLs are replaced by a custom dialog fragment that shows the following warning.

![device-2016-09-07-154936](/uploads/87a9800bde2676b0f2ddf5a894aa04eb/device-2016-09-07-154936.png)

Closes #569

See merge request !311

Refactor ValidationManager and fix some bugs

The validation manager uses a pattern of throwing an exception from within a transaction, then trying to ensure that the transaction gets committed anyway by setting it complete in a finally block. This is a bad idea - if the intentional exception isn't thrown but a later method throws an unintentional exception, the changes up to that point will be committed instead of rolled back.

I reworked the control flow to remove this pattern, and in the process realised that the validation manager was doing a lot of work to provide objects to attach to MessageStateChangedEvents, which are only used by tests these days. So I fixed that and a couple of other issues.

The VALID state is no longer needed and has been removed.

Closes #619

See merge request !313

Introduce a DestroyableActivity for UI result handlers



See merge request !314

Resolve Outstanding Blog Issues

This MR has several commits which fix individual small issues. The biggest commit is a refactoring of the `BlogActivity`, so its `ViewPager` can be used also for the Blog feed.

Closes #637

See merge request !309

Merge branch '610-result-handlers-can-return-results-to-destroyed-activities-fragments' into '637-effect-of-touching-a-post-is-inconsistent'

Check if activity is still alive before returning results to it

Closes #610

See merge request !312

Fixes #635

Fixes #634

by making it only selectable in detail view

Don't crash if an RSS entry has no date

Closes #641

See merge request !310

Show Blog Posts always in the same design

This also adds comments to the blog post detail screen that were previously missing.
It re-uses the existing `BlogPostViewHolder` and its `bind()` methods effectively reducing the amount of code necessary.

Closes #636

See merge request !307

Add one introduction test for modified response message

This is only the first part of #627, but I am putting it up for review already, since the second part will be very similar and issues found here will likely apply to both.

See merge request !306

Fix view recycling issue with author name typeface

Closes #633

See merge request !305

Merge branch '638-illegalstateexception-when-navigating-out-of-combined-feed-and-back-in' into 'master'

Allow Blog Controller listener to re-attach

Closes #638

See merge request !304