Add newsletter form to nothing-personal page (Fixes #15218)

[alexgibson]

One-line summary

Adds a newsletter form to the bottom of /firefox/nothing-personal/.

Issue / Bugzilla link

#15218

Testing

http://localhost:8000/en-US/firefox/nothing-personal/

[alexgibson]

This is ready for code review, but marking as do-not-merge for now until we get sign-off that things are ready.

[reemhamz]

Looks great, works great. r+
I'll leave this for you to merge when you get the final go-ahead 👍🏼

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.89%. Comparing base (dee5f9f) to head (3ce51e5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15223   +/-   ##
=======================================
  Coverage   77.88%   77.89%           
=======================================
  Files         163      163           
  Lines        8480     8483    +3     
=======================================
+ Hits         6605     6608    +3     
  Misses       1875     1875           

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[stevejalim]

Makes sense, but it took a moment to wrap my head around it. Maybe some comment or tweaking a var name would help our future selves?

r+wc