provide a way to register interest in a puzzle

[GoogleCodeExporter]

From Michael Baym: Maybe a "keep me informed" option on puzzles that sets up 
dynamic, puzzle-specific mailing lists, so when someone has a breakthrough 
later on the puzzle, or just wants to be read in on it, they can ping everyone 
who's maintaining an interest in that puzzle?

This would allow someone to register interest so that if a breakthrough was 
had, it would alert the interest parties to come mothing back.

Original issue reported on code.google.com by [email protected] on 24 Jan 2013 at 10:59