Validate that the url of the pass is a valid url

[markusahlstrand]

When you create a new pass in the plugin you can optionally pass a url to the page describing the pass. This will for instance be used if a user clicks a pass on the profile page. Currently this is just a string field, but if it's not a valid url it will cause issues in other backend systems. It would be great if we can ensure that this is a valid URL before the pass is created in WP.

[markusahlstrand]

Ensure that the public url is a valid url