Feature/reader blog detail

This PR adds a "blog detail" view to the Reader, as shown in [this video](https://cloudup.com/i0FsODbjSsq). Blog detail displays posts in a specific blog, with an mshot of the blog as the header. To display it:

* Tap an avatar in the reader post list
* Tap an avatar in the reader post detail
* Tap an avatar in a comment beneath a post
* Tap an avatar in the list of users who like a post

Note that blog detail can only be shown for WordPress blogs - it won't appear for feeds in "Blogs I Follow."

This PR also includes a number of Reader-related fixes, most notably it fixes #1297, which caused blogs to be followed as a feed.