Fixes https://github.com/owncloud/core/issues/8539

I guess that should be `default` and not `defautl`, wondering how this ever worked.

My IDE was so sad about this that it marked the whole file in red and yellow and forced me to fix this.

When sharing an ext storage mount point, it will now use the name of the
mount point instead of an empty string for the target path.

Directory seperator is \ on windows
However we can just check for path != dirname(path) because that is
the same for '' (empty string), '.' and DIRECTORY_SEPARATOR on all OS

To make sure that the dynamic mount config and config classes from
external storages are tested, these are now added into the autotest.sh
test run by enabling the app.

load image via javascript and use $(document).width() to determine the proper side of the image to be returned

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