ScheduledJob was written for much more generality than it ended up being
used for. Currently it is used by send_future_email, and nothing
else. Tailoring the model to emails in particular will make it easier to do
things like selectively clear emails when people unsubscribe from particular
email types, or seamlessly handle using the same email on multiple realms.
The example_user() function is specifically designed for
AARON, hamlet, cordelia, and friends, and it allows a concise
way of using their built-in user profiles. Eventually, the
widespread use of example_user() should help us with refactorings
such as moving the tests users out of the "zulip.com" realm
and deprecating get_user_profile_by_email.
zerver/lib/actions: removed do_set_realm_* functions and added
do_set_realm_property, which takes in a realm object and the name and
value of an attribute to update on that realm.
zerver/tests/test_events.py: refactored realm tests with
do_set_realm_property.
Kept the do_set_realm_authentication_methods and
do_set_realm_message_editing functions because their function
signatures are different.
Addresses part of issue #3854.
