* few small cleanup in html template (avoid empty tbody, put id between
quotes)
* switch to tidylib to validate the html instead of the previous hack
calling an external service (https://html5.validator.nu/)
* remove the previous validator script
* add exception for tidylib on empty <span> (these are required by
bootstrap)
- Switch from base64 to URL encoding for the passing the URL, using the built-in Mako filtering
- Apply HTML filtering to Mako output by default
- Disable HTML filtering for nested templates in adduser, modify, and selfmodify