Use links instead of strings for movie images

[NoelDeMartin]

As reported in matrix by @michielbdejong, images are being written as strings rather than links.

[michielbdejong]

Thanks! I think this applies in three places:

• <#movie> <https://schema.org/image>
• <#movie> <https://schema.org/sameAs>
• <#watchAction> <https://schema.org/object>

The Solid Data Module for Movies can grok both links and strings for those attributes.