Add a link from contributors to their userid

This will be used to allow users to edit their own contributor records.

Requires SQL:
ALTER TABLE contributors_contributor add column "user_id" integer REFERENCES "auth_user" ("id") DEFERRABLE INITIALLY DEFERRED;
CREATE INDEX "contributors_contributor_user_id" ON "contributors_contributor" ("user_id");
UPDATE contributors_contributor set user_id=(select id from auth_user where auth_user.email=contributors_contributor.email) where contributors_contributor.email is not null;
Properly send notifications for objects that don't have an approved field

Preivously we crashed when this happened - but it turned out we had no such
objects in the code. We will in the future.
Make it possible for a contributor to edit his/her own data

Only for users that are already listed under /community/contributors/,
make it possible to edit this data, so we don't have to track it manually.
For obvious reasons, we don't allow editing of the "contributor level"
field...

This also changes the contributors model to send a notification to
the slaves list whenever a contributor record is changed, so we can do
post-moderation if necessary.