[1.x] fix(Mentions): allow renderer to be used without context

[DavideIadeluca]

Changes proposed in this pull request:
Allow post mentions to be used without context. As far as I understand, the same thing has already been done for user mentions in the past:

• flarum/mentions@c9d70a8
• flarum/mentions@344abc0

Reviewers should focus on:
Focus if this PR introduces regression in other parts of the code. I'll also have a quick look and see if I can quickly create a test which tests this.

Screenshot
Not applicable

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: (Remove if irrelevant)
• Related core extension PRs: (Remove if irrelevant)

[imorland]

Looks good to me @DavideIadeluca thanks for putting this together!