perf: speed up post creation time

[SychO9]

Changes proposed in this pull request:
This PR brings performance improvements for the post creation endpoint.

• Visibility checking on the notification syncer (which is expensive) is dropped as it didn't actually fix what it was introduced for and was replaced in the past with visibility checking on the specific notification jobs. (ce6cb58).
• When post has a large amount of mentions, it retrieves each mention individually, this PR eager loads mentions once and uses the loaded collection instead (df4d3ca).
• For visibility checking on mentions notifications, this PR also adds some needed eager loads (8aaa330).
• Right now, with group mentions, user mentions, and post mentions (without taking into account extensions), the endpoint has to do a lot of visibility checking, which is an expensive operation when applied to many users. This PR also moves the logic that triggers mention notification to a queueable job (6e1c22a).

Screenshot

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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)

[askvortsov1]

Haven't tested locally, but code changes look good.

[askvortsov1]

Can we annotate array<what>?