R1:7a79131bf279

Replace old hard-coded URI-based "changes saved" jank with new overgeneralized cookie-based "changes saved" jank

Summary:
Ref T13515. Settings currently has some highly specialized code for rendering "Changes saved." messages. The "saved" state is communicated across a redirect-after-POST by adding `/saved/` to the end of the URI.

This isn't great. It needs a lot of moving pieces, including special accommodations in routing rules. It's user-visible. It has the wrong behavior if you reload the page or…