Made "invitations" association in User model polymorphic.

This enables the following to retrieve a collection with all invitations sent by a given user:

   user.invitations

On the other hand, this retrieves the user that invited a given user (this was already working before this commit):

   user.invited_by
Added several fields (mostly related to invitation features) to the user show view in activeadmin.
Added table with invitations sent to the user show view in ActiveAdmin
Added useful columns to user index view in ActiveAdmin.
Fix error in view.
Added opml import and export information to the user show view in ActiveAdmin.
Added tables to display subscribe_job_states and refresh_feed_job_states in the user show view in ActiveAdmin.