Switch to OOXML for improved interoperability

[juliusknorr]

In recent discussions it turned out that it might be better to use OOXML as the default format to avoid interoperability issues with other software.

Admins can still change in favor of the open standard in the settings but we agreed with @pedropintosilva and @jancborchardt we should change the default

[juliusknorr]

@grnd-alt Something you could pick up, might be a good first dive into some of the richdocuments code base bits