Enhance coverage of update description markup from dnf/PackageKit
Moving this from a downstream bug report:
https://pagure.io/fedora-infrastructure/issue/10986
Bodhi, the thing behind package updates under Fedora, allows the maintainers to use Fedora-Flavored Markdown int he update description (I would link what it means, but the page is kinda stupid and the list of available tags is shown as a modal "dialog" when creating an update, thus it's not easily accessible from the outside).
The markdown does not match appstream tags, neither what the gtk+/Pango markup does.
It would be good to recognize more markdown marks and either discard them (like those for headers ####
) or replace them with something sensible.