Login by Rails session

This modification allows login to CM by a trusted session variable
containing the login name. Make sure to add the following to
initializers/reactor.rb:
:trusted_session_var => 'login' # name of session variable with trusted login name (or nil)

Use case:
- User is not logged in to Fiona GUI but on live site
- Live site uses same user repository as CM to make sure login names match

Note:
- Login is not double checked