Metadata: Add support for XMP sidecar CreateDate and Keywords

[graciousgrey]

As user, I want PhotoPrism to extract information from xmp fields that are written by Apple Photos, so that I can switch from Apple Photos to PhotoPrism without loosing data.

Acceptance Criteria:

• Read the following xmp field:

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#">     
      <rdf:Description rdf:about=""
            xmlns:photoshop="http://ns.adobe.com/photoshop/1.0/">
         <photoshop:DateCreated>2018-01-30T20:30:48-05:00</photoshop:DateCreated>
      </rdf:Description>

[benmccann]

PR for this is here: #1161