Use flags rather than re-deploying the application

[jaspervdj]

There are a few points in time where we want to modify the flow of the application:

1. At some point, we switch over to adding people to the waiting list
2. At some point, the confirmation button is enabled
3. At some point, we want to add a note that the T-Shirts have been ordered
4. ...

We've currently done this by just updating the code and re-deploying, but I would feel better if we could put this behind feature flags, or do it based on the current time.