Label support for pull requests


Rollback Plan
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**If this pull request requires anything more complex (e.g., rolling back a migration), you MUST update this section. Otherwise, delete this note.**

To roll back this change, revert the merge with `git revert -m 1 MERGE_SHA` and perform another deploy.

URLs
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QA Plan
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Run op pull-request, make sure it asks you a question about labels. then check to make sure the label got made.