Set starting background color to match superview.
Updates WPRichTextView with new features.

Adds support for setting the background color of the interior text view.
Adds support for setting a specific set of text options
Implements sizeToFit.
Cleans up some stray comments.
Extends WPContentViewProvider for comments.
Use WPRichTextView instead of DTCoreText view.
Use const for constraints. Adds some padding.
Adopt WPCommentContentViewProvider protocol.
Remove cells delegate protocol in favor of CommentContentViewDelegate

Do not perform unnecessary content manipulation when preparing for reuse.
Make invalidate cached height method public.
Switch to new delegate methods.
Simplify cell reuse identifiers
Manage cells for comments with media attachments.

Keeps a separate cache of cells for comments with attachments.
Refreshes cell size when media loads.
Adds comment.
Sets the default background color back to white.
Adjusts priority on margin constraints to avoid breakage.
Adjusts method order.
Refresh only when view loads, not when it appears.
Merge branch 'develop' of github.com:wordpress-mobile/WordPress-iOS into issues/2168-comments-support-media
Adds support for scrollview delegate method.
Adds option to prevent pending media from refreshing layout.