chore(mentions,emoji): tie autocomplete to editor instance

[SychO9]

Changes proposed in this pull request:
If an extension attempts to use multiple editors on the same page, you face issues with the autocomplete dropdowns since they assume there is only one editor on the page. By tying the autocomplete instance and element to the texteditor instance, this can be improved.

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.