Add a "watch" button to add/change notifications for datasets

[robyngit]

Following the mockups in this comment, create a "Watch" button and add it to the header of the dataset landing page. The watch button should open a modal that allows users to check & uncheck which types of notifications they would like to subscribe to for a given dataset. The modal will first prompt the user to login if not already. We will assume that the user account already has an email address.

This issue is blocked by:

• #1234
• #2755