This introduces 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.