Add keywords field to puzzles

[ylin42]

Add a keywords field for cohesion ed use. This should show up under the summary on the puzzles page, and should be displayed alongside the titles on answers.php.

[ylin42]

Steven did most of this. The only part that's missing is displaying the tags alongside the titles; can anyone fix that part? (I know this will cause lots of repeated tags since we've been putting stuff like [META] in titles manually, but we can fix that later.)