Add a 'pls tell your friends' call-to-action

[nuclearpidgeon]

This extension is literally only useful to current UoM students, so the more of them that can be reached to tell them about this extension, the better.

I'm no longer a current UoM student, so spreading it out to students myself is harder now - it would be good to highlight this problem in a call-to-action on the popup page itself.