default file format change impacts existing servers

[jospoortvliet]

Describe the bug
The change to switch to OOXML by default also impacts existing installations. No matter the merits, changing the default behavior people are used too is not a good idea. If we keep it, we should make sure this only applies on new installations...

[elzody]

A potential fix would be:

1. Does a row for the doc_format key exist in the oc_appconfig table?
2. If not, then we keep using ODF since OOXML-by-default was never explicitly enabled
3. If it exists, then is it set to an empty string?
4. If it's set to an empty string, then we leave it alone and keep using ODF.
5. For everything else, default to OOXML.

Would still need to test this, but maybe @juliusknorr also has an idea.