feat: Add events for Notification read / read all

[imorland]

Changes proposed in this pull request:
Adds events for Notification read and readall actions, for extensions to listen for.

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.

Required changes:

• Related documentation PR: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[davwheat]

Would it be worth also passing the time to the events? There's a chance that any event handlers may wish to use it (for whatever reason), and the time at that moment might not be the same as what was set on the DB table.

[imorland]

Yeah, I guess that could be useful. Didn't think of that as part of the use case I hace in mind, but useful nonetheless.