Implementing Identify OAuth feature

More securely identifies that the user is who they say they are.
Deployment steps:
1. update the meta_mw_id column of the existing user records
   with their corresponding centralauth user ids
   (right now it's using their meta user ids)
2. pull the code after merging this change
3. pip install -e . to get the dependency on jwt
4. make sure the pip install didn't pull in a bad httplib2
5. restart wikimetrics

Change-Id: I72155db39995ee1639b42546c344c146839d7d18