feat(notification platform): Handle multiple providers

[ceorourke]

This is one of several PRs to update the notification settings manager functions to return a mapping of providers to a list of users instead of a list of users (what we had previously) to account for the fact that email is no longer the only means of contacting a user.

Ex: {"email": [<User 1>, <User 2>], "slack", ["User 1, User 3]}

[scefali]

@ceorourke seems like you could use filter here to build output instead of doing it itereatively

[mgaeta]

You should delete the unused functions _get_setting_value_from_mapping, should_user_be_notified, and should_be_participating.

[mgaeta]

looks great