Option for sticky top bar

[cellio]

https://meta.codidact.com/posts/286303

Sticky top bars are controversial (not just here). Some people love them and some people hate them. Currently our top bar is not sticky; the linked post is a request for a user option to make it sticky.

I'm marking this as low priority because part of the utility (on other sites) is seeing new notifications right away -- but we don't have live notifications yet (you have to reload the page). The links could still be useful, granted.