[Dashboard] Ability to decide on email notification frequency

• Switching to a simpler email style which simply tells a user how many outstanding notifications/messages they have, sent to them at their desired interval.

For example, a user may have opted to receive weekly emails from the platform which simply read “You have XX unread messages on _________!” without including any further details, but include a link to open the page. Presumably, this would achieve the objective of letting them know that they have outstanding notifications and also prompt them to login to the platform and clear their messages.