In this commit we just change the upload_avatar_image function to accept
two user_profiles acting_user_profile and target_user_profile. Basically
email param is dropped for a target_user_profile so that avatar's could
be moved lateron to user id based storage.
This needs to be deployed to both staging and prod at the same
off-peak time (and the schema migration run).
At the time it is deployed, we need to make a few changes directly in
the database:
(1) UPDATE django_content_type set app_label='zerver' where app_label='zephyr';
(2) UPDATE south_migrationhistory set app_name='zerver' where app_name='zephyr';
(imported from commit eb3fd719571740189514ef0b884738cb30df1320)