Fixes #12364 - Use NPM+Webpack to handle external assets

This commit introduces NPM and Webpack to handle external assets.
It also provides ES6 support and a live reload dev server that can be
used by running `foreman start` instead of `rails start`.
Do not forget to run `npm install` before starting the server....

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 #15975 - replace instance variable ref in subnet partial

fixes #15847 - change mailer helpers from paths to URLs

Use of path helpers in mailers (with Roadie to rewrite to URLs) is
deprecated in Rails 4 and removed in Rails 5, so prefer URL helpers. To
replace URL rewriting and reduce duplication, default_url_options is now...

Fixes #15771 - add :token to the values returned by the API

Fixes #15718 - Correctly display hash parameters

Lookup values should dispaly the value before type cast for hash type
keys. Rails 4.2.6 introduced a change to the form helpers that only used
the value before type cast for user-provided input, so we need to...

Refs #12193 - Remove remaining cached counters

Fixes #12193 - Stop using counter_cache for host[group] counts

This is aimed to prevent deadlocks caused by Rails' implementation of
counter_cache that is hit when multiple hosts are created or updated at
the same time. An added benefit is that the counters will now display...