Port WebUI changes from f2404de871 to glitch-soc

Port 60df87f6f0 to glitch-soc

Port e0b1e17bd0 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.

Mostly a port of 2ccef52a4f to glitch-soc

Change inspired by 691107263c

Fix a regression introduced by 612b00d1bb

Port ed50c2393a to glitch-soc

Port 89c6d2bd6b to glitch-soc

Port 955d5d36e8 to the glitch-soc flavour

This prepares for a third option that would restrict privacy to
privacyPreference(replyPrivacy, sideArmBasePrivacy)

Port cfeb3beb4e to glitch-soc

Port cdb101340a to glitch-soc,
but without dropping support for regexp filters yet.

Port 64d7a63f18 to glitch-soc

Port bd0791d800 to glitch-soc

See also the following commits:

- 0c3ce41031
- 268d90e810

So far, glitch-soc used history.length to decide whether to call `goBack()` or
go to / in order to not leave the webUI. This made clicking the “Back” button
go to the “Getting started” column instead of going back in the browser's
history when such an action would leave the web UI, but also when:
- The WebUI is refreshed (F5)
- A tab is restored
- The history length reaches its maximum (e.g., 50 in Firefox)

This commit fixes these shortcomings by checking `window.history.state`.
Indeed, we only want to go back in the browser's history when the current
location has been reached from within the WebUI, which only happens via
`pushState` as far as I know. Since browser store the serialized state in
the browser history, this also survives page reload and session restoration.

Port 86efccce2a to glitch-soc

Port 45c9f16f71 to glitch-soc

Port b1938d7853 to glitch-soc

Port 3523aa440b to glitch-soc

Port cbf97c03bb to glitch-soc

Port 9a1a55ce52 to glitch-soc

Port b0664a5e6c to glitch-soc

Port missing part of 6ae70a92c9 to glitch-soc

Port fa04945365 to glitch-soc

Port missing changes from 4e4f1b0dcb to glitch-soc

Port 0128b86d30 to glitch-soc