This fixes#1804 in two ways: First, it removes a <!doctype html> from
the start of the mail, if there is any.
Then, it parses the html itself it quick-xml fails, just stripping
everything between < and >.
Both of these would have fixed this specific issue.
Also, add tests for both fixes.
Normally it happens in addr_cmp function, but update_gossip_peerstates
forms a HashSet of addresses, so they should be lowercased beforehand.
Also adjust the mail_with_cc.txt to test for non-lowercase addresses.
we change the name on the wire as the old Chat-Group-Image header
could not be used on random mails, it was the marker for a "Changed" message,
if we would keep this names, things will fail for exising installations
as messages are dropped and a "Group image changed" message is shown instead.
