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[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 58.73%. Comparing base (e53b72a) to head (248bc82).

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #995      +/-   ##
============================================
+ Coverage     58.70%   58.73%   +0.03%     
  Complexity     7173     7173              
============================================
  Files           574      574              
  Lines         22892    22919      +27     
============================================
+ Hits          13438    13462      +24     
- Misses         9454     9457       +3     

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