Adds tools for going back between user modals.
Displays a list of previously opened user modals when navigating between them through links in modals themselves (such as @ mentions in user bios or links in tweets when viewing them in another modal).
Links can be clicked to go open a modal for that user again while clearing ones that were opened afterwards.
History is cleared when going back to regular columns view.
Makes use of TweetDeck's automatic link handling mechanism.
