Local images are not cached at the resourcesUrl relativePath

[stevengharris]

When specifying a resourcesUrl for local images, new local images should be created within the caches directory at the relativePath for resourcesUrl. For example, if you paste an image in, and resourcesUrl is 'resources' relative to the base URL, then the image src should be 'resources/<UUID created by MarkupWKWebView>.'. Currently it is just '<UUID created by MarkupWKWebView>.'.