add instructions length environment variable

[CollinBeczak]

This is to help make development and testing of the challenge editor faster. The default is 150.
REACT_APP_CHALLENGE_INSTRUCTIONS_LENGTH=150

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (7ddf207) 23.66% compared to head (43054f7) 23.66%.

Files	Patch %	Lines
src/services/Challenge/Challenge.js	0.00%	2 Missing and 1 partial ⚠️
...es/EditChallenge/BulkSchemas/InstructionsSchema.js	0.00%	0 Missing and 1 partial ⚠️
...lenges/EditChallenge/Schemas/InstructionsSchema.js	0.00%	0 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2234      +/-   ##
==========================================
- Coverage   23.66%   23.66%   -0.01%     
==========================================
  Files         648      648              
  Lines       22386    22390       +4     
  Branches     6909     6912       +3     
==========================================
  Hits         5298     5298              
- Misses      14284    14285       +1     
- Partials     2804     2807       +3     

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