Add common consent requirements for use in other applications.
Review Request #9945 — Created May 15, 2018 and submitted
djblets.privacy.consent.common, which provides the
base for common consent requirements that might be useful elsewhere.
Applications can make use of these, subclassing them to provide details
specific to their use. By using these, they'll ensure that the
requirement IDs being used are standard. For Gravatars, this will be
important, as a future change will check these IDs before use.
Made use of these in an upcoming change for Review Board.