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.