Show negated passing atoms as blocking submit requirements

A submit requirement that is unsatisfied because of a veto vote
(-label:Code-Review=MIN) is shown with a red "cancel" icon, while one
that is merely missing votes is shown with the neutral "block" icon.

Other requirements can be blocked in the same way, by something that
is present but must be absent. For example, a requirement like
"-subject:WIP" or "-prefixsubject:Edit" cannot be satisfied by adding
votes; the author must change the commit. These were shown with the
neutral icon, as if waiting for reviewers.

Treat an unsatisfied requirement as blocking when one of its passing
atoms is negated in the submittability expression, either with a unary
minus or with NOT. The veto vote case is a special case of this rule,
and the existing check for it is kept.

Note that this also applies to the default No-Unresolved-Comments
requirement (-has:unresolved), which is now shown as blocking while
there are unresolved comments. These also require action from the
author or the commenter rather than from additional voters.

Release-Notes: Unsatisfied submit requirements with a negated passing atom are shown as blocking
Change-Id: Ie5b938b3d376d736a010e58baf5c25ab09d56c85