Don't resolve services when binding listeners

[franzliedke]

Next part of solving flarum/issue-archive#269

Changes proposed in this pull request:
Replace event subscribers with event listeners in core, where it makes sense.

Exceptions:

• Subscribers without constructor dependencies - These have no cost of dependencies, and are useful for grouping a bunch of listeners that simply operate on models.
  • Flarum\User\UserMetadataUpdater
  • Flarum\Discussion\DiscussionMetadataUpdater
• Policy subclasses - These are a bit more complex, as a base class and lots of logic is involved - haven't thought about the best way to get rid of the subscriber model here.

Confirmed

• Backend changes: tests are green (run php vendor/bin/phpunit).

[franzliedke]

I pushed these (and then some) directly on master. Have gathered enough experience doing all of that for all the extensions. 😉