Add vanity /qr URL aliasing mobile_app redirector

[janbrasna]

One-line summary

The shorter the URL, the more readable the QR code gets. This adds some aliasing to be able to link shorter mobile_app redirector URLs.

Significant changes and points to review

Shorter params could also be used on the original endpoint, there's no restriction.

Tests were added to demonstrate mixing'n'matching the patterns between the approaches.

Issue / Bugzilla link

#765

Testing

pytest -k test_redirects.py
http://localhost:8000/qr
http://localhost:8000/qr?c=foo
http://localhost:8000/fy-NL/qr/?product=quux
http://localhost:8000/de-BE/browsers/mobile/app/?p=klar&campaign=qr-focus (will show store error outside germany unless using android ua)
http://localhost:8000/browsers/mobile/app/?product=focus&campaign=qr-focus (control)

[janbrasna]

Wondering where to document this endpoint alias best so it's not just tribal knowledge. Ideally in some code using it, but there's no consumer yet 😅 🤷

[janbrasna]

Hmh, do we actually need to allowlist these somewhere? Was there a place that would list any permissible args?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.61%. Comparing base (5a152ec) to head (e51382d).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #792   +/-   ##
=======================================
  Coverage   87.61%   87.61%           
=======================================
  Files         156      156           
  Lines       10816    10816           
=======================================
  Hits         9476     9476           
  Misses       1340     1340           

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