Allow multiple client-side runtimes on a single page

Make XBlock client-side runtimes proper classes, so that handlerUrl can
be defined in a per-runtime way, and we can have multiple runtimes on a
single page.

[LMS-1630][LMS-1421][LMS-1517]

lms/static/coffee/src/xblock/lms.runtime.v1.coffee

@@ -0,0 +1,18 @@
+@LmsRuntime = {}
+
+class LmsRuntime.v1 extends XBlock.Runtime.v1
+  handlerUrl: (element, handlerName, suffix, query, thirdparty) ->
+    courseId = $(@element).data("course-id")
+    usageId = $(@element).data("usage-id")
+    handlerAuth = if thirdparty then "handler_noauth" else "handler"
+
+    uri = URI('/courses').segment(courseId)
+                         .segment('xblock')
+                         .segment(usageId)
+                         .segment(handlerAuth)
+                         .segment(handlerName)
+
+    if suffix? then uri.segment(suffix)
+    if query? then uri.search(query)
+
+    uri.toString()