fix(mentions): post mentions with deleted author not rendering

[SychO9]

Fixes #3427

Changes proposed in this pull request:
The issue details the problem and solution very well.
Remember to clear the cache when testing locally.

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.

[askvortsov1]

Makes sense, having this logic separated is cleaner.