Enable ESLint in pre-commit and on Travis CI

[mbeacom]

Description

The goal of this PR is to enable eslint to run via pre-commit and on Travis, to ensure JS styling on already styled files.

Checklist

• linter status: 100% pass
• changes don't break existing behavior
• commit message follows commit guidelines

Affected core subsystem(s)

pre-commit, travis, git

Testing

• Tested the pre-commit with both failing and passing cases
• Tested Travis on mbeacom/web

[owocki]

lgtm

[codecov]

Codecov Report

Merging #624 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #624   +/-   ##
=======================================
  Coverage   33.97%   33.97%           
=======================================
  Files          92       92           
  Lines        5053     5053           
  Branches      584      584           
=======================================
  Hits         1717     1717           
  Misses       3269     3269           
  Partials       67       67

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