Review legit HTML problems reported by the CI mustache linter

While reviewing MDL-65205 it was detected that the mustache linter (that is run by CiBoT) was reporting some warnings/errors that may be legit.

See https://integration.moodle.org/job/Precheck%20remote%20branch/74948/artifact/work/smurf.html#mustache (as example from that issue).

Of course some of the reported problems (specially empty tags) are false positives and can be ignored but others seem legit.

So this issue is about to execute the mustache linter] against all the mustache templates, keep apart false positives and try to fix as many real validation issues as possible.

Also, about to consider this task recurrently (say for every release, or yearly or whatever).

Note that, to execute that linter you need local_ci available locally and its nodejs stuff installed with npm, to get the vnu validator at hand. Then, call to the php linter for every mustache in codebase.

That's it. Ciao