Validate URL for custom URLs and QR reader when app has fixed sites

This issue only needs to be tested in 1 device (Android or iOS). Test default app In the app without any fixed site, go to add a site and click to scan a QR code. Scan a QR code pointing to any Moodle site (the URL must use the app's custom URL scheme, like it's done in user profile in Moodle 3.9.0). Check that the app let's you add the site. Test "branded" app with fixed sites Generate an app with at least 1 fixed site. Go to add a site and click to scan a QR code. Scan a QR code pointing to a Moodle site that's in the list of sites for the branded (the URL must use the app's custom URL scheme, like it's done in user profile in Moodle 3.9.0). Check that the app lets you add the site. Now repeat the test but using a QR code pointing to a Moodle site that isn't in the list of sites for the branded app, but using the same URL scheme. E.g. copy the URL of the QR code from master site and change the URL scheme to the one of your app. Check that the app displays an error. Test branded app with site finder Generate a "branded" app with the site finder and setting onlyallowlistedsites to true in config.json. Go to add a site and click to scan a QR code. Scan a QR code pointing to a Moodle site that's in the list of the site finder (the URL must use the app's custom URL scheme, like it's done in user profile in Moodle 3.9.0). Check that the app lets you add the site. Now repeat the test but using a QR code pointing to a Moodle site that isn't in the site finder, but using the app's custom URL scheme. Check that the app displays an error.