Support fixed URL in config.json

Run the app without setting a siteurl in config.json. Check that the behavior is exactly as before and you can add any site. Add a fixed siteurl without browser SSO to the config.json and run the app. Check that you go directly to the credentials page when: You start the app without sites stored. You delete your last site. You click the "Add" button in sites list. Check that you can login in that site. If local_mobile is installed, check that it's used by the app. Now replace the fixed siteurl with a Moodle that uses browser SSO and run the app. Check that, when going to credentials view, a modal is shown telling that you need to login using browser. Check that you can authenticate using SSO. Go to add a site again and now click cancel when the app tells you to login via browser. Check that username and password are disabled. Check that clicking "Login" shows the same modal again.