Change "subscribe" to "sign up"

[lmcnulty]

Per #2818.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.33%. Comparing base (03afdc3) to head (24292c3).
Report is 3 commits behind head on staging.

Additional details and impacted files

@@           Coverage Diff            @@
##           staging    #2821   +/-   ##
========================================
  Coverage    99.33%   99.33%           
========================================
  Files           12       12           
  Lines         4845     4845           
  Branches        25       41   +16     
========================================
  Hits          4813     4813           
  Misses          32       32           

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[kepae]

I'm happy to merge this. Could you please check the translation coverage?

[lmcnulty]

@kepae Yes, it's covered in all four languages in site/gatsby-site/i18n/locales/{locale}/login.json.