check odt templates before opening/rendering them

[ghost]

As of now, we take data from the database, save them temporarily to disk and open them with libreoffice.

Input data on the database is should not be trusted that much.

There should at least be a check that the data is a libreoffice file. I propose the use of python-magic, which checks for the type of data coming along.

Draft proposal, not ready yet.

[ghost]

I think it is working now, (see my branch) but due to unknown problems with my setup, I cannot test it directly myself.