linting: Start enforcing a basic CSS class naming rule (with lots of opt-outs)

linting: Start enforcing a basic CSS class naming rule (with lots of opt-outs)

Wow, so many opt-outs, and a lot of probably-not-going-to-change ones.

Actually wanted pattern:
* "^mw-…"

Content-related classes we're probably not going to scrap:
* "^wikitable$"

Hard-to-change classes used in lots of skins:
* "^toc$"
* "^toctoggle$"
* "^tochidden$"

Necessary for over-rides/specificity battles:
* "^oo-ui-…"

Probably worth considering changing soon:
* "^client-js$"
* "^client-nojs$"

Some of the exemptions have been applied to deprecated styles (where they're being
renamed); some are for code in entirely deprecated areas themselves; some have the
"wrong" name but sit alongside classes with an appropriate one. And some are, very
clearly, in the wrong repo. Such sigh.

Change-Id: I9b8423d53b6750b5ca79d2f69e111f75c93c5bdc