Markdown templates

[jochenklar]

This PR is the sequel to #1174. It refactors some code and adds code to execute the JS in templates. The feature can be tested with {{test}} in a help text of a question and:

<p>
    TEST!
</p>
<script>
    (function() {
        const elements = document.getElementsByTagName('p');
        setTimeout(function() {
            for (const element of elements) {
              element.style["color"] = "red";
            }
        }, 1000)
    })()
</script>

in local_app/templates/local/test.html and

INSTALLED_APPS += ['local_app']
MARKDOWN_TEMPLATES = {
    'test': 'local/test.html'
}
TEMPLATES_EXECUTE_SCRIPT_TAGS = True

in config/settings/local.py.

@MyPyDavid I am not 100% convinced about the approach in HTML.js, please check. The source is https://macarthur.me/posts/script-tags-in-react/.

[jochenklar]

I rebased to 2.3.0 (and removed some superfluous commits) and added TEMPLATES_EXECUTE_SCRIPT_TAGS to settings instead of the data-execute="true" pattern. Ready for review!

[MyPyDavid]

Ok, tests have passed.