Fix eslint issues

[dtinth]

This PR

• Removes remaining ESLint failures warnings in Shove the whole codebase through Prettier #513

  • Except: “packages/bemuse-tools/src/server.js line 6 – Unable to resolve path to module 'rx'. (import/no-unresolved)” — somehow Yarn does not install it?!?!?!

• Removes the lint Gulp task and replace with a direct call to ESLint CLI.

[codecov-io]

Codecov Report

Merging #517 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #517   +/-   ##
=======================================
  Coverage   83.52%   83.52%           
=======================================
  Files         171      171           
  Lines        5403     5403           
  Branches        1        1           
=======================================
  Hits         4513     4513           
  Misses        890      890

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