fix: getPlainContent causes external content to be fetched

[imorland]

Changes proposed in this pull request:
getPlainContent renders the element using jQuery, which when an image is included in the content, causes this image to be fetched from the server unneccessarily.

Impact can be seen for example when a post excerpt is present, either from flarum/sticky or ianm/synopsis. If several posts are stickied, or if synopsis is enabled, many images or videos, etc are fetched.

Taking advice from @davwheat , I've refactored this util to not use jQuery at all.

Reviewers should focus on:
js is not my strong suit, this appears to work, but please let me know if I've missed something!

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)