Currently, labels are (mis-) used to communicate with the user regarding
the presubmit failure reason. This leads to weird situations where the
Gerrit interface tells "Needs labels: Not-Author-Review,
CI-Build-Failure" which are not labels (and don't look like labels).
The introduced class and variable, SubmitRequirement, contains two
fields describing the issue and an optional label. This allows to
implement the two former examples: the CI doesn't need a label, but the
Non-Author review condition might be related to a Verified label, for
instance.
Change-Id: I83ab3fe52cc689416a26f9d4f91ebafb7337d428
227eb5776f