Adds support for GMs in shared channels (#31403)

* Adds support for GMs in shared channels

* Fix linter

* Remove creatorID from slack call

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Co-authored-by: Miguel de la Cruz <miguel@ctrlz.es>

server/channels/app/post_metadata_test.go

@@ -681,7 +681,7 @@ func TestPreparePostForClient(t *testing.T) {
 		directChannel, err := th.App.createDirectChannel(th.Context, th.BasicUser.Id, th.BasicUser2.Id)
 		require.Nil(t, err)
 
-		groupChannel, err := th.App.createGroupChannel(th.Context, []string{th.BasicUser.Id, th.BasicUser2.Id, th.CreateUser().Id})
+		groupChannel, err := th.App.createGroupChannel(th.Context, []string{th.BasicUser.Id, th.BasicUser2.Id, th.CreateUser().Id}, th.BasicUser.Id)
 		require.Nil(t, err)
 
 		testCases := []struct {