Feature/reader discovery

This PR adds some new features to the Reader and fixes some longstanding problems.

## Blog Preview
Displays posts in a specific blog, with info about the blog (name, description, follower count) and an mshot of the blog above the posts. To display it:

* Tap the avatar section above each post in the reader post list
* Tap an avatar in a comment beneath a post
* Tap an avatar in the list of users who like a post

Note that blog preview only works for wp blogs - it's disabled for feeds (see caveat below).

## Tag Preview
Posts in the reader post list now show the "primary" tag for each post when available. Tapping this displays all public posts with the tag and enables easily following it.

## Redesigned Tag Activity
ReaderTagActivity has been renamed to ReaderSubsActivity and significantly redesigned.

* Switched to a ViewPager for easier movement between pages
* Recommended & followed blogs are now shown, and integrate with the blog preview
* You can now follow a blog by typing its URL (with or without http://)

Note that followed blogs does *not* show feeds you follow (see caveat below).

## Backfill
In the past if you synced the reader then don't sync again for some time, with a popular tag there would often be posts missing between the posts previously retrieved and the posts just retrieved.

This PR resolves this problem to a certain degree by detecting when missing posts may exist and "backfills" them. Note that to avoid excessive bandwidth/battery consumption, there is a limit to how many posts will be backfilled.

## Caveat
You may encounter problems related to following/unfollowing feeds while testing this PR. These problems are due to limitations in our API and have existed for a while, but may be more obvious now. I'm working to resolve them, and once they're resolved I'll submit a new PR with the fixes.

## Issues fixed in this PR:
* Fix #249
* Fix #1183
* Fix #1266 
* Fix #1297
* Fix #1299
* Fix #1313
* Fix #1351