[1.x] [extensibility] feat: allow classes that extends AbstractJob to be placed on a specified queue

[imorland]

Changes proposed in this pull request:
Add a static property to AbstractJob, so that any class extending it may specify which queue it should be dispatched on.

Reviewers should focus on:
All jobs within core and extensions have had their constructor updated to also call parent::__construct(). My understanding that if a 3rd party does not call the parent constructor, it simply means that the $onQueue won't be set, therefore not breaking any compatibility?

QA

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: TBC