Add new "Cada appeal" (saisine CADA) request state

This MR adds a new state option for requests which allows marking them as being under appeal with the CADA. This state currently has no impact on the behaviour of the site, other than displaying specific messages. However, using this new state will allow refining stats, and get a better understanding of the lifecycle of requests. The MR also adds user documentation about how to use this state.

This addresses #53 (closed) and #79 (closed).