Reduce size of usernotes data

This PR attempts to prevent the usernotes page from hitting the size limit by:

1) Removing formatting whitespace from usernotes JSON.
2) Storing only the ID of link notes instead of the permalink. (e.x. "http://www.reddit.com/r/aww/comments/22jqcl/this_is_what_happens_when_you_try_to_eat_beef/" is stored as "22jqcl" and linked as "http://redd.it/22jqcl" in the usernotes popup.