refactor: Extract reload_state module.

This is part of work to break some of our
nastier circular dependencies in preparation
for our es6 migration.

This commit should facilitate loading leaf-like
modules such as people.js before all of the things
that reload.js depends on.

static/js/server_events.js

@@ -131,7 +131,7 @@ function get_events_success(events) {
 function get_events(options) {
     options = _.extend({dont_block: false}, options);
 
-    if (reload.is_in_progress()) {
+    if (reload_state.is_in_progress()) {
         return;
     }