notifications: Extract zerver/lib/url_encoding.py.

Extracting this helper library will help us avoid an import loop
between notifications.py and message.py (with bugdown in between).

But in addition to that, it's a more natural model, since some of the
uses for these functions weren't part of the notifications code
anyway.

zerver/lib/url_encoding.py

@@ -0,0 +1,31 @@
+import urllib
+from typing import List
+
+from zerver.models import Realm, Stream
+
+def hash_util_encode(string: str) -> str:
+    # Do the same encoding operation as hash_util.encodeHashComponent on the
+    # frontend.
+    # `safe` has a default value of "/", but we want those encoded, too.
+    return urllib.parse.quote(
+        string.encode("utf-8"), safe=b"").replace(".", "%2E").replace("%", ".")
+
+def encode_stream(stream_id: int, stream_name: str) -> str:
+    # We encode streams for urls as something like 99-Verona.
+    stream_name = stream_name.replace(' ', '-')
+    return str(stream_id) + '-' + hash_util_encode(stream_name)
+
+def pm_narrow_url(realm: Realm, participants: List[str]) -> str:
+    participants.sort()
+    base_url = "%s/#narrow/pm-with/" % (realm.uri,)
+    return base_url + hash_util_encode(",".join(participants))
+
+def stream_narrow_url(realm: Realm, stream: Stream) -> str:
+    base_url = "%s/#narrow/stream/" % (realm.uri,)
+    return base_url + encode_stream(stream.id, stream.name)
+
+def topic_narrow_url(realm: Realm, stream: Stream, topic: str) -> str:
+    base_url = "%s/#narrow/stream/" % (realm.uri,)
+    return "%s%s/topic/%s" % (base_url,
+                              encode_stream(stream.id, stream.name),
+                              hash_util_encode(topic))