Load balancers or reverse proxies may rewrite or modify the Location
header and unescape its value, which would result in redirects not
preserving the original set of query parameters. This was especially
evident for autologins where we need to redirect to `/oauth2/login` with
the `redirect` parameter containing the original requested URL so that
the end-user ultimately ends up at the latter URL.
We avoid this issue by base64-encoding the original URL, before passing
it along as the intended redirect for the login route.
To preserve existing behaviour, we use a separate query parameter
for the `/oauth2/login`-endpoint that accepts and handles base64-encoded
values.
Replace hardcoded callback URLs with dynamic generation
of URLs based on incoming requests. These are validated against
a pre-registered list of ingresses for which Wonderwall is considered
authorative for.
We also preserve the cookie behaviour; the most specific ingress path
and domain is used for the cookies.
The `url` package has been moved to the `handler` package, and its
implementation refactored slightly for readability and DRY.
