Adds messages for challenge unenroll, decline and approve
Fixes some message center stuff + more

- Removes markdown sanitizer step from views, this is already done when
storing a message, double sanitation leads to visible HTML tags
- Supervisors and participants can now send messages to one another,
participants can also click "Contact Supervisor" on challenge page
- Adds automatic notifications for challenge (un)enrollement and
challenge declined/approved
Merge pull request #208 from boersmamarcel/addmessages

"hotfix" Changes to message center