Saltcorn: Open Redirect in `POST /auth/login` due to incomplete `is_relative_url` validation (backslash bypass)

Summary Saltcorn validates the post-login dest parameter with a string check that only blocks :/ and //. Because all WHATWG-compliant browsers normalise backslashes \ to forward slashes / for special schemes, a payload such as /\evil.com/path slips through isrelativeurl, is emitted unchanged in t...