Add overwritable title and content fields for QR codes

[bluewave41]

One-line summary

Adds an overwritable title and content field to QR codes

Issue / Bugzilla link

https://mozilla-hub.atlassian.net/jira/for-you?tab=assigned&selectedIssue=WT-761

Testing

• Checkout this PR
• http://localhost:8000/cms-admin/pages/321/edit/
• Toggle the QR code and set a custom title and content
• Open http://localhost:8000/en-US/download/ios/ and ensure the custom content displays

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.63%. Comparing base (ef324f3) to head (3370e83).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1635   +/-   ##
=======================================
  Coverage   87.63%   87.63%           
=======================================
  Files         158      158           
  Lines       10843    10845    +2     
=======================================
+ Hits         9502     9504    +2     
  Misses       1341     1341           

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