Fix stored XSS in URL dashboard widget via dangerous URI schemes

Stored cross-site scripting in the URL dashboard widget in Checkmk <2.5.0p5, <2.4.0p31, <2.3.0p48, and all 2.2.0 versions allows a user with dashboard editing permissions to store a URL with a dangerous URI scheme such as javascript: that executes scripts in other users' browsers when they view the dashboard.