Forbid users from creating DMs and GMs with remote users (#28855)

* Forbid users from creating DMs and GMs with remote users

This change prevents the application from creating new DMs and GMs
when remote users are involved, and prevents as well new posts from
being created into preexisting DMs and GMs with remote users created
in previous versions.

* Adds i18n

* Skip preexisting DM tests and create a new one for the new behavior

server/channels/api4/shared_channel_test.go

@@ -161,7 +161,29 @@ func TestGetRemoteClusterById(t *testing.T) {
 }
 
 func TestCreateDirectChannelWithRemoteUser(t *testing.T) {
+	t.Run("should not create a local DM channel that is shared", func(t *testing.T) {
+		th := setupForSharedChannels(t).InitBasic()
+		defer th.TearDown()
+		client := th.Client
+		defer func() {
+			_, err := client.Logout(context.Background())
+			require.NoError(t, err)
+		}()
+
+		localUser := th.BasicUser
+		remoteUser := th.CreateUser()
+		remoteUser.RemoteId = model.NewPointer(model.NewId())
+		remoteUser, appErr := th.App.UpdateUser(th.Context, remoteUser, false)
+		require.Nil(t, appErr)
+
+		dm, _, err := client.CreateDirectChannel(context.Background(), localUser.Id, remoteUser.Id)
+		require.Error(t, err)
+		require.Nil(t, dm)
+	})
+
 	t.Run("creates a local DM channel that is shared", func(t *testing.T) {
+		t.Skip("Remote DMs are currently disabled")
+
 		th := setupForSharedChannels(t).InitBasic()
 		defer th.TearDown()
 		client := th.Client
@@ -185,6 +207,8 @@ func TestCreateDirectChannelWithRemoteUser(t *testing.T) {
 	})
 
 	t.Run("sends a shared channel invitation to the remote", func(t *testing.T) {
+		t.Skip("Remote DMs are currently disabled")
+
 		th := setupForSharedChannels(t).InitBasic()
 		defer th.TearDown()
 		client := th.Client
@@ -217,6 +241,8 @@ func TestCreateDirectChannelWithRemoteUser(t *testing.T) {
 	})
 
 	t.Run("does not send a shared channel invitation to the remote when creator is remote", func(t *testing.T) {
+		t.Skip("Remote DMs are currently disabled")
+
 		th := setupForSharedChannels(t).InitBasic()
 		defer th.TearDown()
 		client := th.Client