Tiny manifest validation issues

[regisrob]

No blocking issues since the "Publish" feature is not yet developed and Mirador does not complain about what follows... but for the sake of validation:

Per IIIF/api#1147, thumbnail should be either a URL string or an object with an @id but in the latter case it should also have a @type (manifest.tpl#L26)

The @type of the image resource should be dctypes:Image (rather than dcterms:Image) (see Presentation API) (manifest.tpl#L38)