History refactor

* makes history into a singleton
* adds a summary of visited sections and (primary) keywords
* removes repeats from history, and instead:
    * moves repeated page to front of list  
    * adds count of # of visits to it 
    * updates it's timestamp
* removes sequence.js !

NB. list is now always in most-recent-first order

@phamann @NathanielBennett