hashchange: Add hash_util.get_hash_category().

This replaces hashchange.get_main_hash(), which had
a slightly misleading name.  Also, moving this to
hash_util forces us to keep 100% coverage on it.

static/js/hash_util.js

@@ -2,6 +2,11 @@ var hash_util = (function () {
 
 var exports = {};
 
+exports.get_hash_category = function (hash) {
+    // given "#streams/subscribed", returns "streams"
+    return hash ? hash.replace(/^#/, "").split(/\//)[0] : "";
+};
+
 // Some browsers zealously URI-decode the contents of
 // window.location.hash.  So we hide our URI-encoding
 // by replacing % with . (like MediaWiki).