Issue/625 notification reader not browser

Fix #625 (among other things)

* Whenever possible, note detail fragments enable showing the associated post or comment natively (reader detail for posts, comment detail for comments)
* Note detail fragments that show a list of people now use a "follow" button with a text label (same as reader liking users)
* Simplified and cleaned-up note-related code, removed unused code and resources
* Note layouts now use Open Sans and have consistent margins, padding, and font sizes
* Fixed bug that caused unescaped entities in user names
* Fixed bug that could (rarely) cause comment detail to show the wrong comment when opened from a notification
* Fixed bug that caused empty user names in reblog notifications

Except where noted above, the UI for notifications is largely unchanged. Rethinking the UI will come in a later PR.