Show "Write a new Post" message when there are no posts

Implementation for #1308.

When there is no `title`, there can't be a post so we can then add the message. The mobile implementation was a bit tricky, right now it uses `position: fixed` which I'm not entirely happy about. On the other hand, duplicating the message in the post list template doesn't sound very good either. Will try to find a better solution, any suggestions are very welcome.