FIX: Retain tags with class='moz-txt-link...' (PR #13176)

Apologies, current workload does not permit me to write tests. This is a fairly simple fix for

where instead of all tags with class=“moz-…” being elided, the ones with class=“moz-txt-link…” are retained (these are links inserted by Thunderbird).


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The title of this pull request changed from “Retain tags with class=‘moz-txt-link…’” to "FIX: Retain tags with class=‘moz-txt-link…’

Sorry we can’t accept it without tests.