Make SSO button use separate form.

[dracos]

This reduces confusion if a browser auto-fills the email form but someone still clicks the SSO button, or similar. [skip changelog]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.48%. Comparing base (e9b3d1d) to head (cf7e084).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5178      +/-   ##
==========================================
- Coverage   82.50%   82.48%   -0.02%     
==========================================
  Files         406      406              
  Lines       31725    31728       +3     
  Branches     5048     5048              
==========================================
- Hits        26174    26172       -2     
- Misses       4060     4065       +5     
  Partials     1491     1491              

Flag	Coverage Δ
	82.48% <100.00%> (-0.02%)	⬇️

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