fix: improve email detection in SSO login form

[atomrc]

Description

see wireapp/wire-web-packages#5591

Screenshots/Screencast (for UI changes)

Checklist

• PR has been self reviewed by the author;
• Hard-to-understand areas of the code have been commented;
• If it is a core feature, unit tests have been added;

[codecov]

Codecov Report

Merging #15953 (cb24e94) into dev (0c530c4) will increase coverage by 0.00%.
Report is 1 commits behind head on dev.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev   #15953   +/-   ##
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  Coverage   44.28%   44.28%           
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  Files         683      683           
  Lines       22919    22918    -1     
  Branches     5213     5213           
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  Hits        10150    10150           
+ Misses      11468    11467    -1     
  Partials     1301     1301