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.
add a fixWidowWord trick in WPTextView
remove non breaking spaces from strings.xml and use the fixWidowWord attribute instead
ignore silver searcher config file
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
Merge branch 'develop' into issue/1880-widow-words
Merge branch 'develop' into issue/1880-widow-words
Merge pull request #1972 from wordpress-mobile/issue/1880-widow-words

fix #1880 by using a simple trick to fix widow words
Merge remote-tracking branch 'origin/develop' into feature/1961-add-mentions-to-commenting