docs.brew.sh: use brew.sh theming.

Add missing `:cask` metadata to specs.

Remove `BUNDLE_PATH` from `brew tests`.

Move Cask specs into `brew tests`.

Use `--force-default-config` option for `brew cask style`.

Add `:cask` to all Cask specs.

Move Cask test helpers to `test/support`.

Remove `dev-cmd/tests.rb` from test coverage.

Use `described_class` in `Tap` spec.

Silence `cmd/cask` test.

Use `Gem.bindir` instead of `#{Gem.dir}/bin`.

Unfortunately there's no easy way to share this code with the brew.sh
site but it's at least able to have the `css/`, `img/` (root) and
`_layouts` directories in sync between the two repositories.

Some small CSS tweaks were needed to make these pages render more
attractively.

java_requirement: don't consider macOS Java stub

Don't check Subversion HTTPS pre-Sierra.

Add exceptions for node@* versioned formulae to use conflicts_with
instead of keg_only :versioned_formula since they, and the main node
formula, all currently overwrite npm during postinstall.

The system Subversion doesn't handle new certificate authorities (e.g.
Let's Encrypt) well enough for this check to be useful.

Update Cask `spec_helper`.

Move Cask `shared_examples` to `test/support`.

Make Cask tests work without tapping `caskroom/cask`.