This is useful in scenarios where the reverse proxy may have knowledge
of user emails, but does not know about usernames set on gitea,
as in the feature request in #19948.
I tested this by setting up a fresh gitea install with one user `mhl`
and email `m.hasnain.lakhani@gmail.com`. I then created a private repo,
and configured gitea to allow reverse proxy authentication.
Via curl I confirmed that these two requests now work and return 200s:
curl http://localhost:3000/mhl/private -I --header "X-Webauth-User: mhl"
curl http://localhost:3000/mhl/private -I --header "X-Webauth-Email: m.hasnain.lakhani@gmail.com"
Before this commit, the second request did not work.
I also verified that if I provide an invalid email or user,
a 404 is correctly returned as before
Closes#19948
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: 6543 <6543@obermui.de>