API: Don't show own posts in mentions, some "vier" changes and provider name for diaspora

These are only some small changes:
- Libertree has problems with horizonal rulers when they don't start with a blank line
- Friendica posts to Diaspora now contain the "provider name" (application name)
- API: The mentions always showed the own posting as well - which was very annoying, because everytime you made a post, you instantly got a notification about a mention. Additionally now only notes are shown, no likes anymore.
- Vier: Attached links are displayed in a slightly different way, making it look more modern - I hope.