Fix multiple assignments to same constants
Move code around
Add virtual attributes to Participant model

Add `Participant#first_arrival_date` and `Participant#last_departure_date` public methods.
Set default dates for new accommodation
Revert "Connect the database at "compile time""

This reverts commit dd92a438136f121002f83947e8eea8e5ef02096d.
Revert "Remove unnecessary 'connect_database'"

This reverts commit f83cae6a9451b1357384a9f226ed97e060980609.