Mentat

As was reported by the then current CSIRTmaster, Mirek Sochor, the report feedback form does not work. The reason is an internal error because a deprecated function was used inside the Jinja template. This is another bug related to migration to Jinja 3.

Here is the removal commit upstream:
https://github.com/pallets/jinja/commit/b0015c72d5acbf93b9d99a1ce6167889338db85b

The argument 'indentfirst' of do_indent() was renamed to 'first'. That is trivial to fix.
Obviously, we must have been ignoring the Deprecation warning. That should be fixed also.