Move to Manifest V3 API

[matatk]

I've had accessibility concerns about this for some time, but if this is not updated by the end of 2022, the extension will stop working, so now's the time to pick up work on it.

• It may be possible to request host permissions on install: MDN: host_permissions (but sounds like this still needs to be handled dynamically).
• Google's guide to migrating to host_permissions in MV3 (the bit about content script matches is a bit confusing).

Tasks:

• [ ] Code
• [ ] Moderator info update
• [ ] ...?

[matatk]

Firefox doesn't support using '3' as the manifest version out-of-the-box yet. This helpful comment has a suggested workaround.

[rsgranne]

It’s been a while & now Google is forcing Manifest v3 on us. Any plans to upgrade this extension so it works with v3? Tks!