adminchannel: rework topic-mask commands

[dgw]

Description

• .tmask get shows the current topic mask
  • .tmask without arguments assumes the get subcommand
• .tmask set now sets the topic mask, and provides a usage hint if called without further arguments
• .tmask clear is added to explicitly clear the topic mask

In combination, the above should be sufficient to resolve #2596.

Checklist

• [x] I have read CONTRIBUTING.md
• [x] I can and do license this contribution under the EFLv2
• [x] No issues are reported by make qa (runs make lint and make test)
• [x] I have tested the functionality of the things this change touches

Notes

If no one minds adding this to 8.0.0 while the release notes are still under peer review, I'm happy to get it shipped sooner.

(Because it's modifying command behavior, if we don't ship this patch in 8.0.0, I believe the semver-appropriate thing to do would be to ship the first commit in 8.0.1 and hold the second for 8.1.0.)

[dgw]

Refactored to use subcommands. I kind of like this style, actually. The .help situation is a bit messy, though the details of that would be a discussion for the sopel-help issue tracker.

[dgw]

Last night, @half-duplex said in our IRC: "reading only the strings and decorators in 2601: lgtm" — and that puts to rest the earlier discussion about .cleartmask vs. .tmask clear or other single-command structure. Shipping… :shipit: