Account for user installations in Channel.permissions_for

Co-authored-by: owocado <24418520+owocado@users.noreply.github.com>

discord/abc.py

@@ -699,6 +699,7 @@ class GuildChannel:
         - Member overrides
         - Implicit permissions
         - Member timeout
+        - User installed app
 
         If a :class:`~discord.Role` is passed, then it checks the permissions
         someone with that role would have, which is essentially:
@@ -714,6 +715,12 @@ class GuildChannel:
         .. versionchanged:: 2.0
             ``obj`` parameter is now positional-only.
 
+        .. versionchanged:: 2.4
+            User installed apps are now taken into account.
+            The permissions returned for a user installed app mirrors the
+            permissions Discord returns in :attr:`~discord.Interaction.app_permissions`,
+            though it is recommended to use that attribute instead.
+
         Parameters
         ----------
         obj: Union[:class:`~discord.Member`, :class:`~discord.Role`]
@@ -745,6 +752,13 @@ class GuildChannel:
             return Permissions.all()
 
         default = self.guild.default_role
+        if default is None:
+
+            if self._state.self_id == obj.id:
+                return Permissions._user_installed_permissions(in_guild=True)
+            else:
+                return Permissions.none()
+
         base = Permissions(default.permissions.value)
 
         # Handle the role case first