The serializer needs to make each mailcite starts from head of a line.
However, mailcite may be a blocked `<span>`. So, its preceding `<br>`
is not required from the HTML point of view, but we need to preserve
it for the serializer.
If we need this hack in some other places, we should make
`HTMLEditUtils::GetFollowingUnnecessaryLineBreak()` aware of this
special handling in a follow up bug.
Differential Revision: https://phabricator.services.mozilla.com/D270784
3d600c8b5d