Add image scraping support

[WithoutPants]

Resolves #344

Adds the ability to scrape performer images and scene cover images.

This change also introduces the subScraper xpath post-processing option. If subScraper appears in an attribute xpath configuration, then the sub-scraper will be executed after all other post-processes are complete. It then takes the value and performs an http request, using the value as the URL. Within the subScraper config is a nested scraping configuration. This allows you to traverse to other webpages to get the attribute value you are after.

For example, from the Boobpedia scraper config in #333 :

...
performerScraper:
  performer:
    # ..snip..
    Image:
      selector: //table[@class="infobox"]//tr[2]//a/@href
      # URL is a partial url, add the first part
      replace:
        - regex: ^
          with: http://www.boobpedia.com
      subScraper:
        selector: //div[@class="fullImageLink"]/a/@href
        replace:
          - regex: ^
            with: http://www.boobpedia.com

This fragment gets the URL from the xpath //table[@class="infobox"]//tr[2]//a/@href, adds the http://www.boobpedia.com prefix with the replace post-process. Then the sub-scraper post-process is run. It requests the document from the resulting URL, then gets the URL from //div[@class="fullImageLink"]/a/@href of the resulting page, followed by the replace post-process.

The Image value is expected to be a URL itself, which the system will subsequent request and encode.

Also adds image scraping to the stash scraper.

[bnkai]

Tests ok with me.
Images are fetched ok and saved with the boobpedia demo scraper.

[WithoutPants]

Rebased and ported UI changes to 2.5. @bnkai can you please review on v2 and v2.5 of the UI?

[MrX292]

stashapp/CommunityScrapers#2 some scrapers for it

[bnkai]

Tested against v2 and v2.5 UI using the boobpedia and @MrX292 's mofos , newfreeones xpath scrapers.
Both scene and performer images seem to work fine.