Welcome action

This allows to use system settings to define the "home" action.
This means a user could land on a CMP when logging in (this could be "overridable" per user, using user settings).

Makes use of 2 system settings : 

* `welcome_action` to define the action "controller" (defaulting to "welcome")
* `welcome_namespace` to define the namespace the "controller" belongs to (defaulting to "core")