Thumbnail generation for PDF files

## Is your feature request related to a problem? Please describe.
Currently ocis does not support thumbnail generation for PDF files.

## Describe the solution you'd like
Being able to generate thumbnails for PDF files. This can be easily done using [imagemagick](https://github.com/gographics/imagick) although that requires cgo to be enabled. Alternatively you can use the imagemagick cli program.

## Describe alternatives you've considered
There is really no alternative.

## Additional context
I'm using the owncloud iOS client