Port 64d7a63f18 to glitch-soc

Port c75493755f to glitch-soc

Port bd0791d800 to glitch-soc

Port e0354aba7c to glitch-soc

Fixes #541.

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 dafd7afc5e to glitch-soc

Port b1938d7853 to glitch-soc

Port 66359ec522 to glitch-soc

Port 306267dbd2 to glitch-soc

Port 0ba49eca8b to glitch-soc

Port 3523aa440b to glitch-soc

Port 2f3ac14a43 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

Port 2587fcdd27 to glitch-soc

Port webui changes from cba2897108 to glitch-soc

Port 22e067bf5c to glitch-soc

Port 1951ff41b3 to glitch-soc

Port SCSS changes from 93c66f0c03 to glitch-soc

Port SCSS from 1e02dc8715

Port SCSS changes from 0f2fbf7d05
(I'm sorry for the animated rainbows)