feat: send notifications of a new reply when post is approved

[SychO9]

Changes proposed in this pull request:
Replies held for approval naturally don't send out notifications to users, however currently after a post has been approved users are still not notified that a reply has been posted.

This PR tweaks subscriptions to send notifications once a reply has been approved.

Reviewers should focus on
The code in approval was extracted into a listener because no matter what listeners are always executed before subscribers even if the extension is set to load before.

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.

[imorland]

Looks good to me!