lint: Fix many no-unused-vars eslint rule violations.

These have been carefully audited by tabbott to ensure they are
unlikely to cause regressions.

static/js/message_flags.js

@@ -41,7 +41,7 @@ function batched_updater(flag, op, immediate) {
         start = _.debounce(server_request, 1000);
     }
 
-    on_success = function on_success(data, status, jqXHR) {
+    on_success = function on_success(data) {
         if (data ===  undefined || data.messages === undefined) {
             return;
         }