fix(a11y): add accessible labels to notification grid options

[davwheat]

Fixes #3369

Changes proposed in this pull request:

• adds a11y labels to the notification grid

Reviewers should focus on:

Is there a better way this label could be written?

Screenreaders will announce, for example, checkbox. receive "someone renames a discussion i started" notifications via web. ticked.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Backend changes: tests are green (run composer test).
• Core developer confirmed locally this works as intended.
• Tests have been added, or are not appropriate here.