Configurable issue labels

Trying the scanner for the first time, I’m very happy with how it works except for one thing: it creates too many labels for its issues. Screenshot of my [sphinx-wagtail-theme setup](https://github.com/wagtail/sphinx-wagtail-theme/issues?q=sort%3Aupdated-desc+is%3Aissue+state%3Aopen+):

That’s just too much noise in our label dropdowns that will interfere with triage of other issues. It’s made worse because with alphanum sorting all those `axe` labels will be at the top. Personally I’d say it’s also too much noise for listings too.

### Proposed solution

Make this configurable. I would prefer to have no labels at all, or otherwise the option to use a label of my choosing at least, like our existing `accessibility` label. My feedback label-by-label:

- `axe-scanning-issue`: make this label configurable. It’s nice those issues are auto-labelled but I think I’d always prefer to use our generic `accessibility` label instead.
- `axe-` rule labels: allow switching on/off. This is very useful information but just way too much noise within label sets used for other types of work.
- `best-practice`: I don’t see myself ever wanting this. It’s useful information but again just not worth the noise in label sets.

My personal preference would be to have all of this off by default so you can make a conscious decision of what labels to use. None of those labels / label creation behavior is currently documented so this threw me off.

The issue does not name files or tests. Start by locating the scanner's label-creation and configuration entry points, then trace how issue labels and rule labels are selected. Done means label creation can be disabled or customized as requested, the default behavior is defined, and the available options are documented.

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