Warning: -convert-to is deprecated.  Use --convert-to instead.
Warning: -outdir is deprecated.  Use --outdir instead.

Limit the size of the string generating the preview image for TXT files

My changes were made against stable7 and I merged them to the file in master, so this version is now the properly merged file for master.
I'm just adding `owncloud-<instanceid>` to the path where temp files are stored.

In order to avoid conflicts between multiple instances installed on one server, it's required to define a LibreOffice configuration folder per instance.

Just like with my PR for the documents app (https://github.com/owncloud/documents/pull/353), I propose to use /tmp/owncloud-instanceid

Use the env:UserInstallation flag like https://github.com/owncloud/documents/blob/master/lib/converter.php.
This will stop the creating of multiple directories for document conversion in tmpDir.

movie previews: use file directly when it's stored locally and encryption is not enabled, fixes #7756

When a picture is encrypted, save it to a temporary file first so that
the PHP function for rotation can access it as file.

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

in some cases 1MB was no enough to generate thumbnail

This prevents excessive logging and disk access

On some distros, avconv isn't available yet.
This fix adds a fallback to ffmpeg.
When none is found, no error message is displayed in the console any
more.

Fixes #5745

Also, this now uses exec() instead of shell_exec() to be able to get the
return code.

The binary path found with "which" is now used to prevent further FS
binary lookup when running the commands.

Moved the "-ss" parameter before "-i" for ffmpeg.