Add rubocop to Rails template

[m-o-e]

Update:
Oops! Sorry, please don't merge (yet).

I just noticed that our opal-dep unfortunately blocks
the latest version of rubocop from getting installed:

Bundler could not find compatible versions for gem "parser":
  In snapshot (Gemfile.lock):
    parser (= 2.3.3.1)

  In Gemfile:
    rails-hyperstack (~> 1.0.alpha1.0) was resolved to 1.0.alpha1.5, which depends on
      opal-browser (~> 0.2.0) was resolved to 0.2.0, which depends on
        opal was resolved to 0.11.4, which depends on
          parser (= 2.3.3.1)

    rubocop (>= 0.67.2) was resolved to 0.68.1, which depends on
      parser (>= 2.5, != 2.5.1.1)

I still think that adding rubocop would be a good idea,
but this PR should not be merged until our opal dependency
has been updated to >= 1.0.0 (which should resolve the conflict).

Sorry for spotting this problem late (after the PR was already up).
In my testing it worked as intended because I have a recent version
of rubocop installed globally (outside the bundle). But this can obv
not be relied on in the installer that is shipped to users.

---

This gives users who start with the rails template
a lint-clean installation and a safe .rubocop.yml.

You can try it out by running:

rails new MyApp -T -m https://raw.githubusercontent.com/m-o-e/hyperstack/8cfe59b4df54b9b947f45ba204b9342da623daae/install/rails-webpacker.rb

This will:

• Perform the regular Hyperstack install
• Run rubocop -a to auto-fix the (minor) linter-complaints that the default install comes with
• Leave you with a lint-clean MyApp and a safe .rubocop.yml that disables all rules that can break Hyperstack, plus tweaks some to allow for Hyperstacky syntax (e.g. multi-line blocks with { })

It does in no way enforce the use of rubocop in the new project
but provides a safe starting point to users who want to use it.

I believe this is important because the rubocop defaults (without a .rubocop.yml)
are not safe for Hyperstack. As of today, when you run rubocop -a on a vanilla
Hyperstack project without this config then it will almost certainly break.

[catmando]

I agree with this 100%. I use rubocop with a custom set of rules no problem. Not sure what the issue is you r referring to

[m-o-e]

Not sure what the issue is you r referring to

Sorry, should've clarified: The latest version of rubocop that our opal dep allows
to be installed only supports Ruby up to version 2.4. I don't think we should ship it that way,
that's why I set the PR "on hold". ;)