Add functionality to produce QR codes

[johannaengland]

First step for #2586.

Not used anywhere yet, just the first step. Can be tested by changing the function itself to save the picture using img.save(path).
I am uncertain about the way of getting the font, but to be able to change the font size we need to get a font from somewhere.

[github-actions]

Test results

     12 files       12 suites   11m 48s ⏱️
3 322 tests 3 322 ✔️ 0 💤 0 ❌
9 441 runs  9 441 ✔️ 0 💤 0 ❌

Results for commit 68a368d.

♻️ This comment has been updated with latest results.

[codecov]

Codecov Report

Attention: Patch coverage is 91.17647% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 60.25%. Comparing base (b1cdf9a) to head (b7f9e65).

Files	Patch %	Lines
python/nav/web/utils.py	91.17%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2887      +/-   ##
==========================================
+ Coverage   60.23%   60.25%   +0.02%     
==========================================
  Files         602      602              
  Lines       44050    44084      +34     
==========================================
+ Hits        26534    26565      +31     
- Misses      17516    17519       +3     

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[lunkwill42]

Works for me, just see some inline suggestions :)