Add loading and success indicators for backend checks

[rebecca-shoptaw]

Closes #9590.

Feature. Adds loading and success indicator icons for API requests in registration form, i.e. to check if username/email is available. Avoids awkward pause as checks run and sudden error message appearance/disappearance when validating on key-up.

Technical

Very simple! Ended up just going with two images, both initially rendered display: none, and then conditionally shown/hidden via the script in realtime_account_validation.js.

Changed a few class names around to be able to apply the password icon styling to all three, and to be able to query the correct icons to show/hide.

Testing

1. Go to /account/create
2. Enter a non-taken username or email address and click onto the next input
3. A spinner should appear right away, replaced by a checkmark when the check is complete
4. Try the same process but this time with a taken username or email address
5. A spinner should appear right away, and when the check is complete it should disappear and an error message should show as usual

Screenshot

Stakeholders

@cdrini

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 63.63636% with 4 lines in your changes missing coverage. Please review.

Project coverage is 17.12%. Comparing base (ce16a79) to head (ba3aa44).
Report is 461 commits behind head on master.

Files with missing lines	Patch %	Lines
openlibrary/plugins/openlibrary/js/signup.js	63.63%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9593      +/-   ##
==========================================
+ Coverage   16.06%   17.12%   +1.06%     
==========================================
  Files          90       89       -1     
  Lines        4769     4752      -17     
  Branches      832      831       -1     
==========================================
+ Hits          766      814      +48     
+ Misses       3480     3428      -52     
+ Partials      523      510      -13     

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[cdrini]

Lgtm! On small accessibility feedback. Otherwise, ran through the page and the indicators appeared when expected, and completing the form correctly created the account!