Set ENABLE_EMAIL_SUBSCRIPTIONS to false by default

Also raise 400 error in user_notifications
view function if ENABLE_EMAIL_SUBSCRIPTIONS is
False.
Set ENABLE_EMAIL_SUBSCRIPTIONS to False in OsfTestCase class

Prevent real calls to the mailchimp api by setting
ENABLE_EMAIL_SUBSCRIPTIONS to False in the OsfTestCase
class.
Move turning off subscribes from OsfTestCase to DbTestCase
Fix test_user_choose_mailing_lists_updates_user_dict

Set settings.ENABLE_EMAIL_SUBSCRIPTIONS to True since
the tests check that subscribe gets called.
Merge branch 'samanehsan-feature/mailchimp' into release/0.22.0
Fix incorrect test