Add mobile app store redirector (Issue #14172)

[alexgibson]

One-line summary

Adds a helper that detects Android/iOS and then redirects to the appropriate mobile app store URL (with optional campaign parameters). Default fallback is to Apple App Store.

Note: the redirect at firefox/browsers/mobile/app/ isn't used by anything yet, but my hope is to use this URL to embed inside QR Codes, as a replacement for Adjust links in #14172

Issue / Bugzilla link

#14172

Testing

• https://www-demo6.allizom.org/firefox/browsers/mobile/app/?product=firefox should redirect to either Firefox on the Apple App Store or Google Play Store depending on user agent.
• https://www-demo6.allizom.org//firefox/browsers/mobile/app/?product=firefox&campaign=firefox-whatsnew should redirect with additional campaign parameters.
• https://www-demo6.allizom.org/firefox/browsers/mobile/app/?product=focus&campaign=firefox-browsers-mobile-focus should redirect to Focus instead of Firefox

[codecov]

Codecov Report

Attention: 22 lines in your changes are missing coverage. Please review.

Comparison is base (ff5010f) 76.93% compared to head (d60a99f) 76.72%.

Files	Patch %	Lines
bedrock/firefox/redirects.py	14.28%	12 Missing ⚠️
bedrock/redirects/util.py	16.66%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14208      +/-   ##
==========================================
- Coverage   76.93%   76.72%   -0.21%     
==========================================
  Files         145      145              
  Lines        7843     7868      +25     
==========================================
+ Hits         6034     6037       +3     
- Misses       1809     1831      +22     

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[alexgibson]

@stevejalim @robhudson if either of you can find some time to review this (since it's mostly python) it would be appreciated? (not urgent though) Thanks!

[stevejalim]

EDIT: r+

---

OLD: r+wc 👍

Works fine, but have a suggestion for simplifying the querystring checking