Cross-site Scripting vulnerability through custom widget URLs and form redirect URLs in grist-core

grist-core is a spreadsheet hosting server. A user visiting a malicious document or submitting a malicious form could have their account compromised, because it was possible to use the `javascript:` scheme with custom widget URLs and form redirect URLs. This issue has been patched in version 1.3.1. Users are advised to upgrade. Users unable to upgrade should avoid visiting documents or forms prepared by people they do not trust.