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.
