1118 Encode strings as JSON in `where` clauses

[dantownsend]

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Resolves https://github.com/piccolo-orm/piccolo/issues/1118

[codecov-commenter]

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Codecov Report

:x: Patch coverage is 80.00000% with 2 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 86.24%. Comparing base (db57d6e) to head (15404b4). :warning: Report is 55 commits behind head on master.

Files with missing lines	Patch %	Lines
piccolo/query/operators/json.py	80.00%	2 Missing :warning:
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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1119      +/-   ##
==========================================
- Coverage   88.42%   86.24%   -2.18%     
==========================================
  Files         117      117              
  Lines        8653     8659       +6     
==========================================
- Hits         7651     7468     -183     
- Misses       1002     1191     +189     

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