fix?: Allow mentions from non-post objects

[imorland]

Fixes #2902

Changes proposed in this pull request:
Extensions such as askvortsov/flarum-moderator-warnings, fof/pages and fof/moderator-notes allow for mentioning a user outside of a Post. This functionality was broken in the 1.0 release due to pulling the user directly from the post relationship.

This is an interim solution, first proposed by @datitisev in #2902, until the refactor of the Formatter API.

I added && $post->exists to the conditional proposed in the issue, as this maintains the current expectation of extensions using the formatter in this way (ie passing new Post() to the renderer.

Examples:
https://github.com/FriendsOfFlarum/moderator-notes/blob/7496beb5abcdec2c2a48505c1c05dc4343831548/src/Api/Serializer/ModeratorNotesSerializer.php#L49
https://github.com/askvortsov1/flarum-moderator-warnings/blob/5be9d5cf272c88262fb0161f3a0b80c7eb445bd2/src/Api/Serializer/WarningSerializer.php#L47

Screenshot
With this change:

Without this change:

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]

Makes sense to me. I wonder if it might be worth separating out the common logic into a separate method.