Port part of 7c48688d85 to glitch-soc

Port front-end changes from f4d549d300 to glitch-soc

Port some parts from 1a7a74ff76

Port cd049454be to glitch-soc

Upstream's implementation has been merged a while ago and is the preferred
way to set fields, as it is the only one compatible with upstream and is
more user-friendly.

This commit deletes the legacy glitch-soc FrontMatter functionality in order
to clean up the code and make it easier to maintain.

Port d185f3ddaf to glitch-soc

This doesn't change anything for glitch-style fields, but those will go
away eventually

glitch-soc-style still in backup, both sharing the same SCSS style

Backport 3987bd18a4 to glitch-soc

Port WebUI changes from 58cede4808