Notification system for items of interest

Simplest mechanism is to show a page when the user logs in which displays comments/edits etc for any item, or related items, to which the user is subscribed since they were last active on the site. So to do this:

• store the last active timestamp of the user in the system (not last login as they might remain signed in)
• allow the user to 'follow' objects on the site, e.g. fragments etc
  Then either:
• query database for new items each time they make an action on the site and display new items on the toolbar - potentially impacts performance as this check is done on every action of the user
• better: a 'feed' where the user sees activity relating to things they are following on a specific 'feed' page meaning this check on new items is only done when they visit that page

The issue names no files, tests, or entry points, so start by reviewing the existing user activity, subscription, and item models before choosing between toolbar checks and a feed page. Done means users can follow relevant objects, activity is tracked since their last active time, and new comments or edits are shown for followed items without checking every action.