Fix some styling issues with posts/new and posts/edit

The title on posts/new was spread over two lines.
The edit form on posts was too small.
Fix issue where blockquotes were being escaped in the text of posts

Implemented the sanitize gem to fix this up. Required a bit of tweaking with the formatters, but we got there in the end.

Fixes #359
Fix permission issue where users who could not read a forum could still post to it
Add missing icon for hidden topics