Use Twig to strip Twig syntax from help topics files in the syntax checker

Problem/Motivation

#3066512: Add checks for syntax and display of help topic Twig template files added a syntax checker which uses regular expressions to remove front matter and certain Twig syntax from the help topics files. Twig, however, has a much more flexible syntax -- for example {% set is just as good and {% set foo="}%" %} will also break the regular expression.

Proposed resolution

Fight fire with fire: use the Twig to render these things into HTML the way we need it.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet