[MM-23727] Make channel validation consistent on the server (#14230)

* MM-23727 Ensure user ids not allowed in channel name:

MM-23727 Move channel name validation to model level

* MM-23727 Update wording

app/channel_test.go

@@ -744,6 +744,14 @@ func TestRenameChannel(t *testing.T) {
 			"",
 			"",
 		},
+		{
+			"Success on rename open channel with consecutive underscores in name",
+			th.createChannel(th.BasicTeam, model.CHANNEL_OPEN),
+			false,
+			"foo__bar",
+			"foo__bar",
+			"New Display Name",
+		},
 		{
 			"Fail on rename direct message channel",
 			th.CreateDmChannel(th.BasicUser2),