This feature isn't really ready yet -- the relevance isn't good, so
the emails aren't a great experience. More work needed; pending that,
just don't send them.
There's already a per-realm setting, which doesn't have a control in
the org settings UI but does suppress it in the per-user settings UI.
Piggyback on that to suppress that UI control when the feature is
disabled at the server level too.
Also cut a comment that hasn't really made sense since the logic was
changed months ago -- the comment originally explained why we sent
digests on Tuesday, Wednesday, and Thursday, and doesn't correspond to
why we dialled back to weekly on Tuesdays.
The digest emails have little in common with the email mirror, beyond
that they both involve email. Give their tests their own file, with a
corresponding name, so it's easy to find this code's tests.
Greg Price