iet-ou/CR21/url-field: Add new URL field to evidence post-type [LACE]

Hi Martin,

And another one! Bug: #21.

I hope this is new field is fairly straightforward. If the following constant is not defined, then the URL field won't be visible anywhere:

```php
<?php

  define( 'WP_EVIDENCE_HUB_URL_FIELD', TRUE );
```

Thanks, Nick