Set baseUrl depending on the environment

[ylemkimon]

Fixes partly #1525.

• If BASE_URL environment variable is set, it takes precedence
  • e.g., BASE_URL=/ npm start
• If on netlify, it is set to /
  • Detects by the presence of CONTEXT environment variable, chosen it as it seems unique to netlify
  • Tested on https://practical-pare-ca0ab7.netlify.com/
• Otherwise, use /KaTeX/

[codecov]

Codecov Report

Merging #1526 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1526   +/-   ##
======================================
  Coverage    84.1%   84.1%           
======================================
  Files          79      79           
  Lines        4561    4561           
  Branches      804     804           
======================================
  Hits         3836    3836           
  Misses        630     630           
  Partials       95      95

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[kevinbarabash]

LGTM. Thanks for the quick turnaround.