Class edits no longer message clients unless you ask

Editing a single class from its detail page — changing the room, the time, or
the teacher — no longer sends anything to booked clients by default. The
"Notify affected clients" choice used to come pre-ticked whenever the class
had bookings, so a quick room swap (for example while testing streaming) could
email everyone on the roster without you choosing to.

Now the edit is always silent unless you explicitly tick who to tell and by
which channels (email, SMS, push) — the same explicit choice the Schedule
tab's quick-edit already offered. You can also choose to send the same change
notice to the class's instructor. When nothing is ticked, the panel says
plainly: "No message will be sent."

Nothing changed about what the messages look like or who is eligible to
receive them — client notification preferences still apply. The activity log
now also records whether an edit asked to notify anyone, and by which
channels.