fixes #14202 - enable select2 in interfaces modal

Reverts commit 44723b9 and re-enables select2 in the interfaces modal,
instead fixing the issue with the NIC table detail lines to read the
network information out of select2 dropdown menus correctly.

fixes #14415 - check CR availability with Fog, not SETTINGS

Simplifies the code for checking compute resource availability, which
can easily be determined with the registered Fog providers instead of
being stored in SETTINGS in app initialisation. Plugins should always be...

Fixes #14458 - use a base class for STI taxable objects

Fixes #14330 - Provide option in hammer to change display type for
libvirt compute resource

Fixes #14401 - Refreshing Host's global status not persisting to the database

Fixes #14341 - List all images for compute resource

Previously, the listing was paginated, but since the table is loaded
via ajax and handled by datatables, the pagination had no effect,
leading to a list limited to the max per page (20 usually).
Also, improved the lookup to only bring the needed data.

Fixes #5816 - allow editing and displaying self via API

fixes #14434 - refactor tokens into concern from observer

rails-observers is still required via audited, but this removes the
usage in Foreman itself and unifies token-related code.

Fixes #14417 - Update Doc link

Fixes #14393 - Using patternfly icons instead of glyphicons