MM-27909: Add manage_shared_channels permission (#15601)

* MM-27909: Add manage_shared_channels permission

We add a new permission to manage shared channels.

It's a channel scoped permission and only the system admin
has that by default.

https://mattermost.atlassian.net/browse/MM-27909

* change to system scoped

* Trigger CI

* Trigger CI

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>

app/app_test.go

@@ -180,6 +180,7 @@ func TestDoAdvancedPermissionsMigration(t *testing.T) {
 		},
 		"system_admin": allPermissionIDs,
 	}
+	assert.Contains(t, allPermissionIDs, model.PERMISSION_MANAGE_SHARED_CHANNELS.Id, "manage_shared_channels permission not found")
 
 	// Check the migration matches what's expected.
 	for name, permissions := range expected1 {