Feature/1563 primary tag in reader

Fixes #1563 

It goes through the tags and use the one with highest post_count as a primary tag for the post (as requested). Then display this tag on reader (both detail and full size).
I'd skip adding this here.  Tags are specific to posts so it doesn't make sense for things like comments and notes to have to implement a tag related method. I'm really iffy about BasePost implementing it. Its probably enough that ReaderPost implements it.