Backend for Automatic Micro Blogs

This MR introduces automatic personal blogs to the Blog client. When a contact is added, her personal blog will also be added automatically. Also, when a new identity is added, a personal blog for that identity is created.

The first commit changes the blog paradigm to short-form blogs and introduces other things that will be useful later in the UI (!214) such as a BlogPostAdded event and the possibility to delete/remove blogs (not the personal ones).

This MR is based on !224 to prevent crashes that are fixed by it. So please review and merge !224 first.

See merge request !223