Update the gettext linter to disallow HTML in strings that are submitted
to Crowdin for translation. Currently this is an XSS attack vector for
us and we have to manually verify that the strings coming in from
Crowdin contain correctly formatted HTML. Removing all HTML from these
strings gets us a step closer to being able to fully automate our
translation process as a human no longer needs to verify these strings.

Strings that have angle brackets (`<`/`>`) in them that are not for HTML
can still be translated by using the HTML entities `&lt;` or `&gt;`.
Please see
https://docs.gitlab.com/ee/development/i18n/externalization.html#html
for details on how to properly use these symbols.

This is going to be followed up by:

1. A clean up of the existing strings that have HTML in them:
   https://gitlab.com/gitlab-org/gitlab/-/issues/228846
2. The addition of a helper to make it easier for developers to work
   with translatable strings that do need to have formatted content in
   them: https://gitlab.com/gitlab-org/gitlab/-/issues/217935

Issue: https://gitlab.com/gitlab-org/gitlab/-/issues/217933