Clean up, format, comment, move around
Remove useless, dead, inconsistent, or wrong code
Fix a small bug
Use sets to keep attribute names for update/create
Use methods for repeated code in route handlers
Add virtual attributes to Participant model

Add `Participant#first_arrival_date` and `Participant#last_departure_date` public methods.
Work on participant views
Allow to manage participant accommodations
Format comments
Fix multiple assignments to same constants
Fix migrations and reorganize the application
Make sure database connexions get closed
Add a remark about student housing at Arsenal