fix(bbcode): highlight.js does not work after changing post content

[SychO9]

Fixes #3794

Changes proposed in this pull request:

• Explicitly applies hljs after the post content has been updated.
• The relevant commit is 8081e9b the second one is just a re-organization of the code.

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]

This seems reasonable as a fix for v1.8, but I think that for 2.0 we should really try to explicitly run hljs in the frontend (or adopt @n-peugnet's approach in the linked issue), so that we can turn on CSP

[SychO9]

Agreed.