fixes #1167 - some setting values are ignored

refs #1050 removing model level authorization from settings table

this seems more trouble than its worth, as settings needs to be
checked/updated every time foreman is starting.

this leads to potenitail issue when login is enabled, or the admin user is missing etc...

fixes #1050 - Foreman settings should not be viewed/edited by non admin user

splitted setting default data from the migration
this allows to add new data and modify its defaults
improved settings UI a bit more
refs #100

fixes #100 - Setting is now a ActiveRecord object

This allows users to manage foreman settings via the UI.

the UI is a bit ugly, but works :)