Stop fetching the messages when getting the cases in Modlog (#4977)

* Use PartialMessage when Case is fetched from Config

* Update docstring

redbot/core/modlog.py

@@ -286,8 +286,11 @@ class Case:
     modified_at: Optional[float]
         The UNIX time of the last change to the case.
         `None` if the case was never edited.
-    message: Optional[discord.Message]
+    message: Optional[Union[discord.PartialMessage, discord.Message]]
         The message created by Modlog for this case.
+        Instance of `discord.Message` *if* the Case object was returned from
+        `modlog.create_case()`, otherwise `discord.PartialMessage`.
+
         `None` if we know that the message no longer exists
         (note: it might not exist regardless of whether this attribute is `None`)
         or if it has never been created.
@@ -311,7 +314,7 @@ class Case:
         channel: Optional[Union[discord.abc.GuildChannel, int]] = None,
         amended_by: Optional[Union[discord.Object, discord.abc.User, int]] = None,
         modified_at: Optional[float] = None,
-        message: Optional[discord.Message] = None,
+        message: Optional[Union[discord.PartialMessage, discord.Message]] = None,
         last_known_username: Optional[str] = None,
     ):
         self.bot = bot
@@ -617,14 +620,7 @@ class Case:
         if message is None:
             message_id = data.get("message")
             if message_id is not None:
-                message = discord.utils.get(bot.cached_messages, id=message_id)
-                if message is None:
-                    try:
-                        message = await mod_channel.fetch_message(message_id)
-                    except discord.HTTPException:
-                        message = None
-            else:
-                message = None
+                message = mod_channel.get_partial_message(message_id)
 
         user_objects = {"user": None, "moderator": None, "amended_by": None}
         for user_key in tuple(user_objects):