Add instructions to install libsodium

resolve #44

As of today, this causes a CSP issue when a notification is removed via 
rails-ujs, because nonce is not updated by turbolinks and rails-ujs use 
previous nonces.

Sometimes, when the first job is executed milliseconds after the insert, 
the db would not return it.
This way, we can safely perform another attempt.

SSL fixes

Checks listing

Checks SSL

Fixed system test with confirmation email

Rename Notifications.delay to .interval

From French to English "interval" is a better translation than "delay".
It's supposed to mean a number of days between two dates.

Support elliptic curve SSH keys for deployment

Shakespeare quote of the day