Rollup merge of #48631 - focusaurus:remember-collapse-setting, r=QuietMisdreavus
Rollup merge of #48631 - focusaurus:remember-collapse-setting, r=QuietMisdreavus

Remember state of top-level collapse toggle widget

This change allows the big top-right expand/collapse toggle to remember its setting across navigation or page reloads. Prior to this change, there was this annoyance:

- browse to some docs
- Click the minus button to collapse them
- browse to other docs (or reload the page)
- Everything is expanded again

The solution is based on storing a simple boolean flag in localStorage. I think it's a good improvement, but it does introduce the following potentially surprising behavior:

- browse to some docs
- click the minus button to collapse them
- click to expand a particular item (not the main top-right big one)
- reload the page, everything is collapsed

Paired with @debugsteven on this.

GBGamer/rust,aidancully/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,GBGamer/rust