Fix an open redirect exploit where if you went to <https://danbooru.donmai.us/login?url=//fakebooru.com>,
then after you logged in you would be redirected to https://fakebooru.com.
This was actually fixed by the upgrade to Rails 7.0. `redirect_to` now
raises an `UnsafeRedirectError` on redirect to an offsite URL. Before we
tried to prevent offsite redirects by checking that the URL started with
a slash, but this was insufficient - it allowed protocol-relative URLs
like `//fakebooru.com`.
Add a test case for protocol-relative URLs and return a 403 error on an
offsite redirect.
37f2d5925f