Add support for custom feed URIs

[shgysk8zer0]

Need the ability to add subscribe URLs for custom domains with known paths.

When a service is selected that requires a custom domain, a new <input type="url" required /> should be added.

There should also be a means of verifying the the specified domain in fact hosts the service (for Ownlcloud, check GET /status.php).

When such a custom service is selected, the subscribe URL should be selected using new URL(servicePath, customDomain) and setting searchParams accordingly.