[Workflow notifications] Control time when notification goes out

[andybroomfield]

Currently it will go out periodically (daily / weekly / monthly) based on the time the module was first enabled.
Ideally we would like to be able to set a time when it is processed eg. First thing in the morning so it's top of the inbox.

Job Scheduler should be able to do this if we can't do this in the module.

[stephen-cox]

It's easy to set a time for this to happen if it's daily, but the UX will need some thinking about for more complex sending patterns.

One option could be to set a starting time which when set or changed sets the time the configured period starts at. Although this could be a little confusing.