hashchange: Add hash_util.get_hash_section().

We'll use this mostly for streams/settings URLs at first.

frontend_tests/node_tests/hash_util.js

@@ -82,6 +82,31 @@ run_test('test_get_hash_category', () => {
     );
 });
 
+run_test('test_get_hash_section', () => {
+    assert.equal(
+        hash_util.get_hash_section('streams/subscribed'),
+        'subscribed'
+    );
+    assert.equal(
+        hash_util.get_hash_section('#settings/your-account'),
+        'your-account'
+    );
+
+    assert.equal(
+        hash_util.get_hash_section('settings/10/general/'),
+        '10'
+    );
+
+    assert.equal(
+        hash_util.get_hash_section('#drafts'),
+        ''
+    );
+    assert.equal(
+        hash_util.get_hash_section(''),
+        ''
+    );
+});
+
 run_test('test_parse_narrow', () => {
     assert.deepEqual(
         hash_util.parse_narrow(['narrow', 'stream', '11-social']),

static/js/hash_util.js

@@ -7,6 +7,18 @@ exports.get_hash_category = function (hash) {
     return hash ? hash.replace(/^#/, "").split(/\//)[0] : "";
 };
 
+exports.get_hash_section = function (hash) {
+    // given "#settings/your-account", returns "your-account"
+    // given '#streams/5/social", returns "5"
+    if (!hash) {
+        return '';
+    }
+
+    var parts = hash.replace(/\/$/, "").split(/\//);
+
+    return parts[1] || '';
+};
+
 // Some browsers zealously URI-decode the contents of
 // window.location.hash.  So we hide our URI-encoding
 // by replacing % with . (like MediaWiki).