There is a hard to reproduce race condition causing these tests to
occasionally fail. We believe it is caused by switching to the home tab and
not properly waiting for all the messages to load; see Issue #1243. The
tests are for the following pathway (not a high priority to test):
1. User starts editing a message.
2. allow_message_editing is turned off for the realm (in this case, by the
user going to the admin page and turning it off).
3. User finishes editing the message and hits send.
This is controlled through the admin tab and a new field in the Realms
table. This mirrors the behavior of the old hardcoded setting
feature_flags.disable_message_editing. Partially resolves#903.