Improve forum UI

Improvements of the forum interface

Done (#1048)

Make front-end for pinning threads
Tiny profile pic next to the username
Improve styling (see comments in #601 (closed))
User email addresses should not be shown (solved by #929)
Fix delete button in forum thread (not showing for some reason)
Fix delete button in forum thread post (not showing for some reason)

TODOs

Make usernames clickable Solved by: #1112
Show user profile image in forum Solved by: #1113
"Add new post" button should be at the bottom, not at the top of the page? Solved by: #1116