User Preferences Workflow - notifications

[Harmonym]

Need to add a workflow that enables users to opt-out of e-mail, SMS, or all notifications. Perhaps as a settings drop-down from the username at the top right.

[stevejgordon]

This might relate to an issue #1906 I have just broken out from #1084

For requesters this is not easy. We could add something for volunteers though. Would that be useful?

I'm not sure we can always limit to a specific messaging type as sometimes only email or only SMS would be valid. But we could capture the preferences and try to apply them.

Also, we thought it might be useful for SMS to have quiet hours when SMS is not sent. However we need to decide if we queue instead. So needs some discussion.