[Signup] Fix username error bug; Add pw error checks

[imnasnainaec]

Fixes bug with the username field:

• click/tab into the username field
• don't type a username or type one that is too short (at most 2 characters)
• click/tab away from the username field
• observe the red error on the field
• return to the username field and add a username at least 3 characters long
• click/tab away from the username field
• observe that the red error persists, even though it shouldn't

Adds on-blur error checking to the password fields.

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This change is 

[codecov]

Codecov Report

Attention: Patch coverage is 23.07692% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 75.00%. Comparing base (63c6d71) to head (8afb4ab).

Files	Patch %	Lines
src/components/Login/Signup.tsx	23.07%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3067      +/-   ##
==========================================
- Coverage   75.05%   75.00%   -0.05%     
==========================================
  Files         269      269              
  Lines       10378    10387       +9     
  Branches     1228     1227       -1     
==========================================
+ Hits         7789     7791       +2     
- Misses       2227     2234       +7     
  Partials      362      362              

Flag	Coverage Δ
backend	83.95% <ø> (ø)
frontend	66.80% <23.07%> (-0.08%)	⬇️

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[jmgrady]

Reviewed 1 of 1 files at r1, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @imnasnainaec)