zulip: Add hash_util_decode() to decode server encoded URL excerpts.

This adds hash_util_decode() to decode a hash_util_encode() [present in
zulip/zulip's zerver/lib/url_encoding.py [1]] encoded string.

The intent is to facilitate code sharing among various python clients
(primarily, Zulip Terminal).

The string replacement before the `unquote` is to recoup for the custom
string replacements in zulip/zulip's zerver/lib/url_encoding.py [1].

Test added.

[1] See hash_util_encode() in https://github.com/zulip/zulip/blob/master/zerver/lib/url_encoding.py.

zulip/tests/test_hash_util_decode.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python3
+
+import unittest
+import zulip
+
+from unittest import TestCase
+
+class TestHashUtilDecode(TestCase):
+    def test_hash_util_decode(self) -> None:
+        tests = [
+            ('topic', 'topic'),
+            ('.2Edot', '.dot'),
+            ('.23stream.20name', '#stream name'),
+            ('(no.20topic)', '(no topic)'),
+            ('.3Cstrong.3Ebold.3C.2Fstrong.3E', '<strong>bold</strong>'),
+            ('.3Asome_emoji.3A', ':some_emoji:'),
+        ]
+        for encoded_string, decoded_string in tests:
+            with self.subTest(encoded_string=encoded_string):
+                self.assertEqual(zulip.hash_util_decode(encoded_string), decoded_string)
+
+if __name__ == '__main__':
+    unittest.main()