Create mockups for notification system UI

[robyngit]

Create mockups to demonstrate how users will subscribe to the portal and dataset events described in issues #213, #1762, #2250.

Possible UI elements:

• A "subscribe/follow" button on portals and dataset landing pages
• A view that allows users to set the config options for a particular notification subscription, such as the frequency of notifications and the email address to use (if not always the email of the user that's logged in). We may want to allow a user to have fine grained control over each event, e.g. following only citation events and not view events. This view could be embedded in a page or perhaps used in a modal window.
• A view that gives a user a summary of all their current notifications, with the option to unsubscribe or change preferences related to notifications