Allow to have multiple links per public calendar

Adding a public link is still the old way:

```xml
<o:publish-calendar xmlns:o="http://calendarserver.org/ns/"/>
```

Removing all public links is just like we used to unpublish
```xml
<o:unpublish-calendar xmlns:o="http://calendarserver.org/ns/"/>
```

We now have the following for unpublishing a specific link
```xml
<o:unpublish-calendar xmlns:o="http://nextcloud.com/ns/">urltounpublish</o:unpublish-calendar>
```

The public URLs are exposed this way:
```xml
<x1:publish-urls xmlns:d="DAV:" xmlns:x1="http://nextcloud.com/ns/">
	<d:href>urltopublish</d:href>
	<d:href>secondurltopublish</d:href>
</x1:publish-urls>
```

`publish-url` is still available and give the first URL
```xml
<x1:publish-url xmlns:d="DAV:" xmlns:x1="http://calendarserver.org/ns/

">
	<d:href>urltopublish</d:href>
</x1:publish-url>
```

Signed-off-by: Thomas Citharel <tcit@tcit.fr>