Migrate BlogController and FeedController to ViewModel

This uses two different view model instances for blog and feed.

Test instructions:

• create test data with blog posts
• enter main blog feed and individual blogs, rotate screen
• add contact that writes blog posts and reblogs existing posts
  • check that new posts produce notifications when not in blogs (or in other people's blogs)
  • check posts get added to blog list if open when post arrives
• write/reblog blog posts yourself, check that it gets added to main blog feed

Closes #1866 (closed), #1891 (closed)