Migrate send-to-device widget to post directly to Basket (Fixes #11945)

[alexgibson]

One-line summary

Updates Send-to-Device widget to post directly to Basket from JS

Significant changes and points to review

I removed product as a hidden form field and macro parameter, as this doesn't seem to be used anywhere (as far as I can tell?). I believe this may have been included originally for an option where the user can select a product to send a link to, but this has never been used / fully implemented.

Issue / Bugzilla link

#11945

Testing

Demo: https://www-demo5.allizom.org/

• http://localhost:8000/en-US/firefox/welcome/4/
• http://localhost:8000/en-US/firefox/welcome/6/
• http://localhost:8000/en-US/firefox/browsers/mobile/?xv=legacy (test both forms at top / bottom)
• http://localhost:8000/en-US/firefox/browsers/mobile/android/
• http://localhost:8000/en-US/firefox/browsers/mobile/ios/
• http://localhost:8000/en-US/firefox/browsers/mobile/compare/
• http://localhost:8000/en-US/firefox/mobile/get-app/
• http://localhost:8000/en-US/firefox/whatsnew/
• http://localhost:8000/en-US/firefox/60.0/whatsnew/ (need to be signed into FxA)

[codecov[bot]]

Codecov Report

Merging #12028 (fbd6944) into main (851b333) will decrease coverage by 0.09%. The diff coverage is 80.95%.

@@            Coverage Diff             @@
##             main   #12028      +/-   ##
==========================================
- Coverage   77.45%   77.35%   -0.10%     
==========================================
  Files         147      147              
  Lines        7757     7747      -10     
==========================================
- Hits         6008     5993      -15     
- Misses       1749     1754       +5     

Impacted Files	Coverage Δ
bedrock/base/middleware.py	61.53% <0.00%> (ø)
bedrock/firefox/forms.py	94.11% <ø> (-0.89%)	:arrow_down:
bedrock/firefox/urls.py	95.45% <ø> (ø)
bedrock/newsletter/views.py	84.58% <0.00%> (-1.99%)	:arrow_down:
bedrock/privacy/urls.py	100.00% <ø> (ø)
bedrock/firefox/templatetags/helpers.py	93.75% <82.35%> (-1.36%)	:arrow_down:
bedrock/firefox/views.py	91.94% <100.00%> (+<0.01%)	:arrow_up:
bedrock/settings/base.py	93.94% <100.00%> (+0.05%)	:arrow_up:

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[alexgibson]

Thanks for taking a look @stevejalim!