Implement N-up label printing

[d-buchmann]

Make label printing more flexible by allowing to put multiple labels onto a single PDF page in a grid ("N-up printing").
This way you can use e.g. A4 sheets with pre-cut labels and print them in your household inkjet/laser printer - no need for a special label printer.

The grid of the sheet is set by xcount and ycount which I added to the form (but not yet to the database).
You can also set a skipcount which skips the given number of labels in case you have partly printed sheets from previous prints (feature as described/requested in #577).
The size and position tweaking can be done in the additional css to fit any label sheet type.

As far as I have tested, it doesn't break the original behavior using the default values xcount=ycount=1, skipcount=0.
This is a draft because no translation or db migration exists yet.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 59.87%. Comparing base (eb7aefb) to head (f8042c9).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #995      +/-   ##
============================================
+ Coverage     59.83%   59.87%   +0.03%     
  Complexity     6052     6052              
============================================
  Files           529      529              
  Lines         20628    20655      +27     
============================================
+ Hits          12343    12367      +24     
- Misses         8285     8288       +3     

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