Add Fx News one-click-subscribe page (Fixes #15142)

[alexgibson]

One-line summary

Adds one-click confirmation landing page for Firefox newsletter

Issue / Bugzilla link

#15142

Testing

First get your newsletter token by following these steps:

1. Open https://www.mozilla.org/en-US/newsletter/recovery/ and enter your email.
2. Find the recovery email, and click the link that opens the email preference center.
3. Open Dev Tools -> Storage -> Cookies and find the cookie called nl-token. Copy it's value (which should be a long alpha-numeric string).
4. Whilst you're here, make sure you are not subscribed to the Firefox newsletter (unsubscribe if you are).

Next, open the confirmation page using your token in the URL:

1. Open: http://localhost:8000/en-US/newsletter/firefox/confirm/{INSERT_YOUR_TOKEN_HERE}
2. When the page loads, click "Confirm".
3. Wait to see a "thank you" message and then you should now be subscribed to Firefox news.
4. Finally, confirm the subscription has worked by going back to the newsletter management page, and you should hopefully see Firefox News is checked in the list.

[codecov]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.86%. Comparing base (2daa613) to head (290bed4).

Files with missing lines	Patch %	Lines
bedrock/newsletter/views.py	25.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15197      +/-   ##
==========================================
- Coverage   77.88%   77.86%   -0.03%     
==========================================
  Files         163      163              
  Lines        8480     8484       +4     
==========================================
+ Hits         6605     6606       +1     
- Misses       1875     1878       +3     

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