Lint doc-assets like the customizer in testing

[stuartpb]

This should let Travis-CI catch bugs in pull requests to things like the customizer, like #10632 and #10633.

The new rule excludes minified scripts like Respond.js, which I imagine don't lint properly.

[stuartpb]

And of course the docs-assets are full of linting errors (many coming from inlined minified scripts). So this needs further revision to pass the new tests.

[mdo]

For the time being I don't think we need to lint these things, especially since they aren't meant to be used by the community. Open to hearing more thoughts on why this makes more sense beyond the Customizer, something I've long wanted out of our main docs and repo.

[stuartpb]

Well, they are used by, and contributed to by, the community just as any other component of the docs (I'm talking mostly about the Customizer here).

[stuartpb]

Actually, it appears docs/assets is linted in the current test suite (as of 2fc2fe7?)

[cvrebert]

Overturned in #11739.

[mdo]

@stuartpb Yeah, the tooling fell into place and overall I'm really regretting it at this point. It makes for super small assets when we concatenate things, but having to kill the server and run grunt dist as we do today just to change some docs CSS is just awful.

I think I conflated the linting with the above though, so at some point I plan on undoing some of the docs asset management and instead just keep things as simple as possible. This overhead really fucks with productivity.