fix: simplify the way we handle ok status

[wheresalice]

This PR simplifies ok statuses to True/False

• Make is_ok a boolean
• Add warning_message_sent_at (timestamp), displaying in the admin screen and only sending a message if this is None
• Add overdue_message_sent_at (timestamp), displaying in the admin screen and only sending a message if this is None
• Add sos_received_at (timestamp), displaying it in the admin screen

is_ok defaults to true, and is set to False by the healthcheck detecting them as overdue or them sending SOS.

Some additional admin actions have been added to the Members model:

• delete_checkins which removes a member's checkin
• mark_ok which marks a member as ok, along with clearing warning_message_sent_at, overdue_message_sent_at, and sos_received_at

State Model.pdf

[wheresalice]

If it's just a case of changing the text in the admin screen then that's just adding a verbose name, which I've just done.

It is more fiddly to change the actual name of the field.

[kimadactyl]

Hmm I feel like I'd want the tests to reflect safe/unsafe and the messages to admins. So the field could stay the same name but the text used to display it not? Is that a lot?

[wheresalice]

Updated tests and display text to use safe/not safe instead of ok/not ok