feat: add draw-io extension

[JammingBen]

Moves the draw-io extension from the Web repository to here since it has an external dependency and shouldn't be part of the default stack of Web apps.

refs owncloud/web#11248

[kulmann]

It also lets you import some visio file type... not sure how good that works though.

[mmattel]

Will draw-io drop out of the default delivery of ocis (web)?
We have made an announcement in the rolling release notes that is is part of.
In case, how to "install" it from an admins pov?

@tbsbdr fyi because we need to mention in upcoming release notes if that is gonna be changed

[kulmann]

Will draw-io drop out of the default delivery of ocis (web)?

Yes. draw-io uses https://embed.diagrams.net internally. If you want to use draw-io you either need to accept that or deploy your own draw-io server. That's why I don't want to have it enabled by default...

This repo here will have a draw-io app release by the end of the week with a zip file attached. We have documentation in place in the dev docs what to do with that zip file. See https://github.com/owncloud/awesome-ocis

Has been clarified, nothing to change (for now)