3564 Add Twitter sharing for bookmarks

http://code.google.com/p/otwarchive/issues/detail?id=3564

We didn't provide a way to share bookmarks on Twitter. Now we do!

This also DRYs up the share code a bit by putting it all in one view file instead of maintaining separate versions for works and bookmarks. The helper methods continue to be probably more complicated than they need to be, but at least they are now in their own helper file instead of in the works helper.

This incorporates the fixes for [2718 unsecure javascript link for Twitter widget](http://code.google.com/p/otwarchive/issues/detail?id=2718) and [3565 Share pop-up from bookmarks: tumblr post has wrong link](http://code.google.com/p/otwarchive/issues/detail?id=3565), so I'm going to close those pull requests.