Fix thread sorting

cf410d0 broke thread sorting by interfering with the (admittedly
arcane) sort apparatus.

There is a scope.watch in the AppController which is responsible for
filling the `predicate` field of the `sort` object when the `name` field
changes. In order for this to work, the controllers in child scopes
(namely StreamController or WidgetController) need to modify the `name`
property of the `sort` object in the parent scope, via prototypal
inheritance, rather than replacing the entire `sort` object.

h/static/scripts/app-controller.coffee

@@ -33,6 +33,9 @@ module.exports = class AppController
     # Check to see if we are on the stream page so we can hide share button.
     $scope.isEmbedded = $window.top isnt $window
 
+    # Default sort
+    $scope.sort = name: 'Location'
+
     identity.watch({
       onlogin: (identity) -> $scope.auth.user = auth.userid(identity)
       onlogout: -> $scope.auth.user = null

h/static/scripts/stream-controller.coffee

@@ -51,7 +51,7 @@ module.exports = class StreamController
     fetch(20)
 
     $scope.isStream = true
-    $scope.sort = name: 'Newest'
+    $scope.sort.name = 'Newest'
     $scope.threadRoot = threading.root
 
     $scope.shouldShowThread = (container) -> true

h/static/scripts/widget-controller.coffee

@@ -11,7 +11,6 @@ module.exports = class WidgetController
      streamer,   streamFilter,   store,   threading
   ) ->
     $scope.isStream = true
-    $scope.sort = name: 'Location'
     $scope.threadRoot = threading.root
 
     @chunkSize = 200