In T339268 we propose an inline script to replace the existing system and support multiple client preferences. This requires changes to Vector namely:
* We need to use a class suffixed with -clientpref-0 or -clientpref-1 instead of -disabled or -enabled
* We store in a different cookie
# Requirements
[] As an anonymous user the limited width will persist across pages / page refreshes
[] Must be backwards compatible - all preferences saved via the cookie system will work in the new system
[] Merge https://gerrit.wikimedia.org/r/c/mediawiki/core/+/932819 at the same time.
# QA
As an anonymous user:
[] Check the limited width toggle feature bottom right works (persists across page views). Clicking it should expand the article to full width and it should persist on page reload.
[x] (likely done by engineer) check the new CSS/JS works with old HTML
# Sign off steps
[] Create task for monitoring the performance implications of the change
[x] Create task for cleaning up code for cached HTML (Done in T343843)