(#1853) Hostgroups cannot be listed if filter == null 

http://theforeman.org/issues/1853

"There seems to be no easy way to configure a user such that they can see a list
of all hostgroups with out ticking all the host groups in the filter and maintaining that."

This pull requests adds a boolean field on users so that they can subscribe to new hostgroups instead of maintaining the list of hostgroups every time a new one is added. This is tedious and can be automated easily with this pull request, letting an user with view permissions be linked to every hostgroup. As an use case, we have a 'viewer' user that needs to be in all hostgroups so that we do stuff with it through the api. A comment on this ticket also mentions the same problem of having to add all hostgroups to the filter (bad maintainability).

Discussion time :)