Fix a regression caused by jQuery removal (`renderPreviewPanelContent`)
And simplify the file editor, it doesn't need to be that complex. And
remove jQuery code.
None of the frontend js/ts files was touched besides these two commands
(edit: no longer true, I touched one file in
61105d0618
because of a deprecation that was not showing before the rename).
`tsc` currently reports 778 errors, so I have disabled it in CI as
planned.
Everything appears to work fine.
