Restrict HTML in students' input.

cli/answertests_docs.php

doc/en/Authoring/Inputs.md

locallib.php

question.php

questiontype.php

site_overrides/partials/nav-item.html

stack/cas/keyval.class.php

stack/cas/parsingrules/998_security.filter.php

stack/maxima/inequalities.mac

stack/maximaparser/corrective_parser.php

stack/maximaparser/utils.php

tests/input_algebraic_test.php

tests/input_string_test.php