LG-3480: Add ERBLint to forbid deprecated classes

[aduth]

Why: As a developer, I expect that static analysis tooling will prevent me from adding deprecated classes, so that I'm not contributing against the effort to transition from BassCSS to USWDS.

Includes classes marked in LG-3261 technical research as not being in use, plus alignment classes migrated as of LG-3484 (#4540).

An important caveat is that ERBLint is only able to detect classes assigned through a class="..." HTML assignment, and not via a Rails tag helper (e.g. image_tag(..., class: '...')).

[zachmargolis]

LGTM, for the tag helper maybe we should add something that runs in specs, that overrides the actionview tag method (or whatever the base one that they all go through is) and warns/errors if it sees a banned classname passed?

[mitchellhenke]

This is great 👍🏼

[aduth]

for the tag helper maybe we should add something that runs in specs, that overrides the actionview tag method (or whatever the base one that they all go through is) and warns/errors if it sees a banned classname passed?

I like this idea. Incidentally I happened to be looking at the other ticket in this sprint LG-3483 "Implement deprecated class matchers to fail in test specs" trying to recall how it differed from this one. I believe the intention with the other ticket is similar to what you're describing, i.e. incorporate deprecated class detection in the test specs. As originally written, I think it was expected more as hooking in to render or Capybara::Session.visit, but I think the tag method approach could work just as well too (probably a fair bit more efficient as well).