Restore :assets group in Gemfile

[tom93]

TODO: #250 (comment)

Some of those gems (especially mini_racer) are very large, and it is useful to be able to skip them. They were already in a commented-out group; restore the group to allow skipping them (they will still be installed by default).

Note that putting them in a group will mean they won't be auto-required when config/application.rb calls
Bundler.require(:default, Rails.env), but that should be fine.

Also remove the old comment "not required in production environments by default" because it is unclear/inaccurate (we currently run the asset precompilation on the production server), and add hints on how to skip the group.

[coveralls]

coverage: 37.131%. remained the same
when pulling 07ea3cd on gemfile-assets-group
into b26315e on master.

[Holmes98]

I'm not quite sure I fully understand why, but the assets group was removed in Rails 4:

• https://edgeguides.rubyonrails.org/upgrading_ruby_on_rails.html#upgrading-from-rails-3-2-to-rails-4-0-gemfile
• https://stackoverflow.com/questions/16406204/why-did-rails4-drop-support-for-assets-group-in-the-gemfile
• rails/rails@49c4af4

And this doesn't seem ideal, but we might be compiling some assets live in production mode; not sure if that could cause issues with this:

nztrain/config/environments/production.rb

Lines 59 to 60 in b084e51

	# Don't fallback to assets pipeline
	# config.assets.compile = false

[tom93]

Good catch, what a mess. Just wanted to skip some gems...
I'm tempted to keep the assets group (even though Rails got rid of it), because that seems like the way to allow those gems to be skipped.
I'll definitely first double check that asset precompilation still works identically (some of those comments about needing to patch the rake task and missing initializers got me worried).

we might be compiling some assets live in production mode

I hope that can be dealt with separately? (The gems will still be installed in production, just won't be auto-required by default; I'll try to confirm that live compilation still works with that change.)

Feel free to merge this if you don't find any issues.