Change the "Watch this space" icon on a page to a three-state button

Currently the 'watch this space' icon on each page is confusing, not the least because it's the opposite meaning to the favourites icon - With favourites, it's an "on/off" switch, whereas for the watch link it's an icon that describes the state you're not in.

We should try the following.

Make the icon a three-state switch.

1. envelope with grey cross (grey because red would draw too much attention to the icon) indicating the page isn't being watched.
2. envelope with a green tick indicating the page is being watched
3. envelope with a globe indicating the space is being watched

Clicking on the icon will transition it between all three states.

Because un-watching a space is an action that goes beyond the scope of the current page, we should pop up a Javascript "Are you sure?" alert if the user transitions away from state 3, but ONLY if that was the state of the icon when the page was first loaded (so we don't get annoying alerts from someone just clicking through from 2 back to 1)