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

after being uploaded in .part files

Fix #8131

The write-hook also is triggered for created files

Fix #8131

Notice to myself: Stick to my IDE.

getMountByStorageId and getMountByNumericId return an empty array on error,
which should be detected to avoid possible errors. This commit also adds in
some new logging points and throws to aid debugging

Also displays the untrusted domain warning in English