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/zulip/__init__.py

@@ -1574,3 +1574,15 @@ class ZulipStream:
 
     def flush(self) -> None:
         pass
+
+def hash_util_decode(string: str) -> str:
+    """
+    Returns a decoded string given a hash_util_encode() [present in zulip/zulip's zerver/lib/url_encoding.py] encoded string.
+
+    Example usage:
+    >>> zulip.hash_util_decode('test.20here')
+    'test here'
+    """
+    # Acknowledge custom string replacements in zulip/zulip's zerver/lib/url_encoding.py before unquoting.
+    # NOTE: urllib.parse.unquote already does .replace('%2E', '.').
+    return urllib.parse.unquote(string.replace('.', '%'))