Make all jinja {{ interpolation }} safe for MarkDown

[saraedum]

Basically, whenever we use an interpolation, we would have to escape the output for MarkDown, i.e., write something like {{ md_escape(variable) }}. That's very tedious. Since jinja already escapes HTML automatically (I believe) there is probably a way to tell it to also escape MarkDown when variable is a string (and not a Markup object.)

[saraedum]

Apparently, autoescaping in jinja is somewhat configurable. We should enable it for MarkDown escapes.

Additionally, we should apply a render filter to all interpolations so that everything renders as MarkDown if possible. See pallets/jinja#503 (comment) for an approach to automatically apply a filter.