fixes #1170 - adds additional charts to statistics page

this patch adds: hostgroups, compute resources, location
and organizations if they are enabled.

Fixes #12634 - New HW Model flag pxe_loader

Implementation of

https://github.com/theforeman/rfcs/blob/master/text/0001-PXE-Booting-UEFI.md

In short, new host/hostgroup flag PXE loader is added to specify TFTP
DHCP filename and Foreman now orchestrates all PXE-capable templates...

Fixes #16044 - Load dashboard widgets via ajax

This speeds up dasboard loading as widgets are loaded in the background
and do not block rendering, as well as allow the dashboard to load
correctly even if some of the widgets are broken.

Fixes #16238 - Speed up dashboard loading time.

I've change report host to inner query and used a join, in
my testing, this speed up dramaticilly dashboard loading
time, on my setup from 60 seconds to under 10 seconds.

fixes #16172 - remove base_class override in template subclasses

On Rails 5.0, the base_class is used during model instantiation to
validate STI relationships as it now casts similarly to Foreman::STI,
causing failures when base_class returns the subclass rather than parent...

refs #13768 - add random_name call to create_hosts permission

Fixes #15924 - Implement server groups and scheduler hints

Fixes #13768 - random and MAC-based name generator

fixes #15919 - statistics page is loaded via AJAX

Also:
- refactor how statistics data is being called, now it should be trivial
to add additional charts or consume other data from it. It should make
it much easier to reuse statistics charts in other places...

Fixes #14301 - Grant access to help pages for view permissions