URL-based branch changes

[jamesdabbs]

Spun out from #89, since I think it's going to be a little bit more work. Our ideal here would be:

• A sharable/linkable way to reference "the viewer, with data loaded from ..."
• Clicking around the site continues to view the same branch
• Re-visiting the site later uses the previously stored branch (or main)

Probably some investigation needed here. I'm unsure whether this'll be implementation-wise easier with a ?branch=... query param or an optional /branch/{branch}/ base path. @StevenClontz – do you have a preference one way or the other?

[StevenClontz]

Very very slight preference for the query string appended to the end over the prefixed route.

I said something wrong in the other thread: it's perfectly fine if the branch is always main unless the route/query string explicitly says to use an alternative branch. Probably strictly better: if I'm viewing an alternative branch I can share the URL I'm actually at with anyone and they see the same branch.