Remove Souhait in create Wish

The new model only allows to be marked as available or not available. But it
will allows to have preferences on every DPS. Benevole will be able to express
some preferences.
DPS: no plural form for DPS
url: fix parameter name
DPS admin: show the number of users not available
DPS: add a link from the admin to the normal view
DPS: update the number of not available when needed