url_decoding: Add 'is_same_server_message_link' function.

This prep commit adds a lib function 'is_same_server_message_link'.

This will be currently used while compressing quote and reply
in push notifications and later can be used at other places.

zerver/lib/url_decoding.py

@@ -0,0 +1,26 @@
+from urllib.parse import urlsplit
+
+from django.conf import settings
+
+
+def is_same_server_message_link(url: str) -> bool:
+    split_result = urlsplit(url)
+    hostname = split_result.hostname
+    fragment = split_result.fragment
+
+    if hostname not in {None, settings.EXTERNAL_HOST_WITHOUT_PORT}:
+        return False
+
+    # A message link always has category `narrow`, section `stream`
+    # or `dm`, and ends with `/near/<message_id>`, where <message_id>
+    # is a sequence of digits. The URL fragment of a message link has
+    # at least 5 parts. e.g. '#narrow/dm/9,15-dm/near/43'
+    fragment_parts = fragment.split("/")
+    if len(fragment_parts) < 5:
+        return False
+
+    category = fragment_parts[0]
+    section = fragment_parts[1]
+    ends_with_near_message_id = fragment_parts[-2] == "near" and fragment_parts[-1].isdigit()
+
+    return category == "narrow" and section in {"stream", "dm"} and ends_with_near_message_id