Clean up the controller action to send webconf invitations

A lot cleaner now and doing only what it should do: deal with the parameters
submitted by the user and creating a proper response to the user.
Moved more code to Mconf::Invitation.
Better structure for mailers, for now just for webconf invitation

With this structure we can split the Notifier in more mailer classes and
nest their views in app/mailers/views. For now only the web conference
invitation mail has been taken out of Notifier and moved to a new mailer.

refs #1179