Adds a header label to WPContentView.

If a the comment detail view is using a note content provider, add a header to the top that provides some context about the comment being viewed.

Fixes #1444.
Fixing frame property access.
Use horizontal padding constant to match up with the other views.
Merge branch 'develop' into issue/1444-comment-context
Use existing controller methods to load web view, so they will provide credentials for private sites.
Set header label frame's autoresizingMask to fix overflow on iPad.
Moved header label to CommentView subclass.
Removing an extra line break.
Use a constant for the label's vertical inset size.
Merge pull request #1526 from wordpress-mobile/issue/1444-comment-context

Adds a header label to CommentView.