Unify labelName and authorUsername graphql arguments

There is a difference in labelName/authorUsername param names for issues/epics resolvers, I think current status is:

           default listing         | negated                 | unioned
issues     labelName (array)       | labelName (array)       | labelNames (array)
epics      labelName (array)       | labelName (array)       | labelName (array)
issues     authorUsername (single) | authorUsername (single) | authorUsernames (array)
epics      authorUsername (single) | authorUsername (single) | authorUsername (array)

Recently I added singular unioned arguments for epics (singular to be consistent with existing epic params), but this adds difference for unioned filter compared to issues. Ideally we should unify these to be same.