Refactor Bulk Tagger client-side code

[jimchamp]

Refactors some of the Bulk Tagger code. Changes:

• Client-side Bulk Tagger code has been moved to /js/ile
• BulkTagger class created to encapsulate client-side Bulk Tagger code
• Change form's hidden inputs to use type="hidden" (as opposed to hiding them via CSS)
• Prevent fetching subject suggestions if the search term is an empty string
• Use hidden class to hide the Bulk Tagger
• Remove ?debug=true from the bulk tagging form's action
• Use correct tag_subjects default in handler
• Replace unused id in "Tag selections" anchor with an href

Technical

Testing

Confirm that the bulk tagger is working as expected.

Screenshot

Stakeholders

@mekarpeles

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 2.32558% with 126 lines in your changes missing coverage. Please review.

Project coverage is 16.03%. Comparing base (1ac93f7) to head (01df0f5).
Report is 1656 commits behind head on master.

Files with missing lines	Patch %	Lines
...enlibrary/plugins/openlibrary/js/ile/BulkTagger.js	2.43%	111 Missing and 9 partials ⚠️
...ibrary/plugins/openlibrary/js/ile/utils/subject.js	0.00%	5 Missing ⚠️
.../js/ile/utils/SelectionManager/SelectionManager.js	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8444      +/-   ##
==========================================
+ Coverage   15.97%   16.03%   +0.06%     
==========================================
  Files          86       86              
  Lines        4695     4677      -18     
  Branches      822      816       -6     
==========================================
  Hits          750      750              
+ Misses       3422     3406      -16     
+ Partials      523      521       -2     

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