[full-ci] allow to disable previews in webdav & web

[dragonchaser]

We added an env var OCIS_DISABLE_PREVIEWS to disable the thumbnails for web & webdav via a global setting.
For each service this behaviour can be disabled using the local env vars WEB_OPTION_DISABLE_PREVIEWS (old)
and WEBDAV_OPTION_DISABLE_PREVIEWS (new).

refs #192

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

[ownclouders]

💥 Acceptance test Core-API-Tests-ocis-storage-7 failed. Further test are cancelled...

[ownclouders]

💥 Acceptance test Core-API-Tests-ocis-storage-3 failed. Further test are cancelled...

[kulmann]

I'm a bit surprised by thise change. The web option only prevents thumbnails in order to give the possibility to speed up the file list rendering. It does not block previews in the right sidebar details panel. And most importantly the "preview" app which is responsible for rendering media files in a single folder also uses the thumbnail service (using higher resolutions) to render the images relatively big and with high resolution.

Disabling the thumbnail service alltogether seems a little too much for me. Also, I don't share the expected behaviour in the ticket.

[kobergj]

Code looks good, just some description suggestions. But @mmattel is more qualified to judge that 😉

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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