Design for the notifications settings page

[hedgefield]

Splitting this off from #1, this is the place to discuss the design of the notifications settings page.

I couldn't find a whole lot of specs for this in the requirements document, but at least the user should be able to choose which notification 'sources' can actually send notifications, and where those should be delivered (within WordPress, email, push to app/text, etc). Similar to how you can manage this kind of thing in iOS or Android.

I did a first pass at a design for this here: https://www.figma.com/file/PIRT7PBfCYIxn4urohVInn/WP-Notify-design-v2-Tim?node-id=232%3A24

The list of notification sources there should probably be a table, and the wordpress/mail/phone toggles can be improved a bunch in the a11y area alone, but I just wanted to throw an idea down that we can iterate on. Haven't seen a lot of discussion or definition about the elements on these pages, or where they should live in the admin, so let me know what you think!