Resolve "Maintenance mode: Add the possibility of unlocking the migration"

Closes #90 (closed)

current implementation (to discuss) :

when migration is locked and admin clicks on checkbox to disable maintenance mode, he is asked if he wants to re-enable migration

• if response is no, maintenance mode is removed but migration remains locked (if server restarts, maintenance mode will come back)
• if response is yes, migration is unlocked, and a new attempt is made to migrate to latest database version. after migrating, a check is performed to verify if migration failed again (and re-enable maintenance mode if needed).

other possible implementations :

• have separate buttons to unlock/re-run migration.
• don't re-run migration and/or don't check migration status after unlocking (in this case, migration and checks will run again only when server is restarted).

How to verify:

• run app in dev mode and lauch 'meteor mongo' in a separate terminal
• db.migrations.update({},{$set:{locked:true}})
• restart app (or use .reload command in 'meteor shell')
• in laboite app, go to administration -> Réglages administrateurs
• uncheck 'Mettre le site en maintenance'