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

Merge branch '632-badtokenexception-if-rss-error-dialog-is-show-after-activity-is-destroyed' into 'master'

Only show RSS Feed Import Failed Dialog when Activity lives

Closes #632

See merge request !303

Fetch RSS feeds via Tor

This patch replaces jsocks with our own minimal SOCKS 5 implementation, which is compatible with Android's OpenSSL hacks (see discussion on #599 for the horrifying details). This allows us to use OkHttp over Tor to fetch RSS feeds.

It turns out that SOCKS 5 without authentication is a really simple protocol: https://tools.ietf.org/html/rfc1928

Closes #599.

See merge request !308

This also adds comments to the blog post detail screen that were
previously missing.

UI for reblogging and blog comments

This MR wraps all blog posts inside a `CardView` and adds a reblog button to blog posts. When this button is pressed, a new activity opens that shows the blog post again and allows the user to reblog it into their own blog. An optional comment can be added.

Also a new compound view `AuthorView` is introduced to display authors and their trust level in a consistent way.

The `BlogController` has been refactored, so the `FeedController` can share most code through an abstract base class.

![reblog2](/uploads/ce4764851c4cbe39684c95a3b427e3a2/reblog2.gif)

Closes #496, #437 

See merge request !300

Closes #593

Merge branch '605-add-database-method-for-retrieving-a-contact-by-local-and-remote-author-ids' into 'master'

Add database method for retrieving a contact by local and remote author IDs

Closes #605

See merge request !302