Migrate preview system to external preview server systems

[LukasReschke]

Transferred from an older security tracker ticket to make it public. Ref https://github.com/owncloud/security-tracker/issues/156

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Industry standard, pointed out already at https://github.com/owncloud/security-tracker/issues/85 in the general considerations.

This is even advised upstream, see: https://github.com/mkoppanen/imagick/tree/131c1238140aacb82883d8808cc33402dd1b70a3#security:

Because ImageMagick is used to process images it is feasibly possible for hackers to create images that contain invalid data to attempt to exploit these bugs. Because of this we recommend the following:

1. Do not run Imagick in a server that is directly accessible from outside your network. It is better to either use it as a background task using something like SupervisorD or to run it in a separate server that is not directly access on the internet.

[rullzer]

I think this would be a very good idea!

As a bonus this would make the system more extendible (want an obscure preview format... write an app that does it).

[LukasReschke]

Writing apps is actually already possible :)

[pierreozoux]

What about the "recommendations" from: https://imagetragick.com/
It wouldn't be enough?

[MTRichards]

If I read this correctly, the recommendations would be enough for the short term, but I think this solution from @LukasReschke is also to make the overall system less prone to these sorts of problems in the long run.