[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:

• https://github.com/flarum/mentions/commit/c9d70a844c0a9eb8e9c5d63eb2f7fee3185ac370
• https://github.com/flarum/mentions/commit/344abc0f87a1d1514f650b247f9d7511b5245088

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!