Rounds all WPNetworkImageViews with `AVATAR` type.

Rounds gravatars in Comments, Notifications and the Reader.
Added border around noticon to match design.

Circularization of avatars is now processed in an AsyncTask tom improve performance.
Should have used `executeOnExecutor()` for the circularize bitmap task.

Props @nbradbury
Added `SITE_AVATAR` enum to keep blavatars square.
Merge branch 'develop' into issue/1958-round-avatars
Merge pull request #1970 from wordpress-mobile/issue/1958-round-avatars

Avatar Circularization