UI - Sidebar: Categories (and Collapsability) for fava-sidebar-link.

[seltzered]

For those using the fava-sidebar-link feature, when adding more than a few sidebar links the layout can get crowded such that a user cannot see core aspects like the 'errors' sidebar link & color indicator.

There's probably a number of possible enhancements (re-arrangements, etc.), but would like to suggest the ability to have a category field for fava-sidebar-link entries that would render sidebar categories that could be collapsible.

[yagebu]

This is not something that I'd implement but a PR would be welcome.