Enhance user Posts cells UI to be similar to ReaderPosts

The boring cells for user posts has been enhanced to be very similar to what has been done in ReaderPosts. The differences between them are the action buttons (edit/delete), a full date and a status label instead of follow button. Since Posts and ReaderPosts are very similar now, a lot of logic has been moved to abstract classes. Here are a few things worth mentioning:
* AbstractPostTableViewCell and AbstractPostsViewController has been introduced as a superclass to relevant classes of both Posts and ReaderPosts
* "author" is now stored for Posts
* A summary has been created for Posts to be similar to ReaderPost instead of using the raw text with html tags
* Deleting a post shows an alert view explaining the post will go to trash
* Contains a lightweight migration for adding authorAvatarURL to Posts

As a side note, I haven't been able to test featured images completely as I don't have any blogs returning that. I actually don't know how an image becomes a featured image. Having said that, I did test it with a static image url and it's working fine. It'd be great if it can be tested with a blog before merging in the changes.

closes [#2022](https://github.com/wordpress-mobile/WordPress-iOS/issues/2022)

We don't have any good conventions for const names yet, but lately more people are trying to prefix them just in case, e.g. `WPPostSummaryLength`.