merge and generalize lint rules

[willkg]

The linter is kind of a mess right now. Issues:

1. We have "lint rules" and "template lint rules", but we should really have "rules for id strings" and "rules for translated strings". In other words, one file with all the rules in them and they're all of type LintRule. That'll reduce a ton of repeated code.
2. Lint rules do goofy things to make sure they only look at strings that apply to them. For example, some lint rules only apply to strings with python-format variables. Right now, we do goofy things at the beginning to handle that. We should do something better. Maybe call the lint rule with more context so it's easier for it to figure out whether it should check things?
3. The documentation for lint rules is split between the docs and the code and it's almost certainly out of sync. We need a better way to deal with that. Maybe document it in the code and extract it with autodoc?
4. We should have better registration infrastructure for lint rules. Right now it's pretty hard-coded-esque and not plugin-friendly.