Add proper checks for XML/HTML tags

[mathjazz]

We currently don't enforce any XML/HTML checks.

There's Translate Toolkit checks: http://docs.translatehouse.org/projects/translate-toolkit/en/latest/commands/pofilter_tests.html#pofilter-tests-xmltags

But they can be disabled and bypassed, and errors & warnings are never stored into the DB. We also don't run any checks on PO files on sync, file upload or submission through batch actions.

We also have HTML checks in the external linter: https://github.com/mozilla-l10n/mozl10n-linter/blob/main/l10n/android_l10n.py