Fixes #14492 - Unify delete and minimize for dashboard widgets

Originally the idea was for the buttons to have a different effect on
the widgets, however with changes introduced later on these differences
have been reduced to being insignificant.

Fixes #18726 - Set null to true on migrations for Rails 5

Fixes #18426 - Add actions to notification entries

This adds support for the notifications to display actions. In case no
actions are provided, the dropdown kebab isn't shown. The actions
possible right now only include GET links.

Fixes #17228 - Select templates as global defaults (#4283)

This should allow user to select templates that will be used
for building PXE Default on TFTP proxy. These templates will
not be updated from seeds during updates.

Refs #18576 - update OpenSUSE mirror url

Fixes #16982 - Scope properly when no taxonomies are set

The default scope for hosts and other objects did not restrict
properly by taxonomies. An user without organizations or
locations, could do anything it's permissions allow to.
The list of hosts was unrestricted and showed hosts in...

Fixes #16204 - Singularize ExternalUsergroups for permissions

fixes #18481 - host index page loads faster

Fixes #18440 - removes dead code and obsolete permission

Fixes #18321 - added migration for builtin roles