Images on public sharing get downscaled to increase use experience - this will speed up loading time

- adding keep aspect to core/ajax/preview.php
- remove duplicate method Preview::show()
- no more hard coded mimetype of preview
- remove .png from the preview urls
- keep old route preview.png for backwards compatibility
- aspect preserving previews are now cached
load image via javascript and use $(document).width() to determine the proper side of the image to be returned
Merge pull request #8407 from owncloud/scale-pics-on-public-sharing-master

Images on public sharing get downscaled to increase use experience - thi...