Refactor Django REST Framework API again + clean up AngularJS directive

Refactor to include still only include two (2) API endpoints, but
this time, 'api-battle-list' for the list of all Battles and
'api-battle-player-list' for the list of Players for a given Battle
(no Battle specific data included).  JSON data restructured as lists
of records, the list being the value of the single key-value pair in
the final resultant JSON dictionary with the key "results".