Redirect deleted branches to main branch

[wjt]

A frequent pattern we see is:

1. Alice creates a branch B
2. Alice sends Bob and Christine a link to the build of branch B
3. Christine approves the changes, merges the branch and deletes it
4. Bob says, “hey, that link is broken, where did it go?”

We could add a stub page at /index.html where each deleted branch used to be, explaining the situation and (if the branch was merged (is this easy to determine?)) offering a link to the root site (preserving the URL hash and query string).

This requires a semi-permanent record of which branches have ever existed but we kind of already have this: artifacts (expired or not) persist with a record of which branch they came from even after the branch is deleted.