Comment user block design tweaks:

 * Timestamp moved to below user name
 * Site name or url is shown next to timestamp
 * Site name/url is tappable and loads site preview or url in a web view.

Fixes #1957
Simplified display of user's site url.

If a user has no title but a site url, display that in the user block.
Embolden the header user's name.
Added null check on `getOnNoteBlockTextClickListener()`
Merge pull request #1959 from wordpress-mobile/issue/1957-comment-user-tweaks

Tweaks to notifications comment user display