Add feature flag for emails and add link to account.mako

Show link to notifications setting page if
settings.ENABLE_EMAIL_SUBSCRIPTION is True.

Add link to account.mako in notifications.mako
and vice-versa.
Syntax fixes
Use find_one to query for user instead of find

Find_one will throw NoResultsFound error, not find
Add error_type='not_subscribed' to unsubscribe error

Check against this error_type in view model and
show error message for all other errors.
Change MAILCHIMP_WEBHOOK_SECRET_KEY default to None
Rename mailchimp_helpers to mailchimp_utils in test mocks
Remove index from User.find_one query in sync_data_from_mailchimp
Set default of MAILCHIMP_API_KEY to None