Add one test
Adds
Adds challenge model test
Fixes coverage to 100% for challenge model
Adds coverage for user
Adds messages for challenge unenroll, decline and approve
Fixes coverage for challenge decorator
Fixes days count
Draft look of challenge tweaked
Fixes days count
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
Added db:migrate
Fixes issues

#216 and #217