New image viewer

[Cyperghost]

Closes #5966

TODO

• [x] Remove language variables wcf.imageViewer.*
• [x] Check that .jsImageViewer is no longer used
  • [x] Check that data-caption and not title is used
• [x] Use shared_imageViewer instead of imageViewer
• [ ] Rebuild WCF.Combined.min.js and WCF.Combined.tiny.min.js
• [ ] Add documentation how to use the new image viewer
  • [ ] Add migration guide from old image viewer