Build: Added ESLint.

[EricDunsworth]

• Updated package.json dependencies.
• Added an "eslint" task.
• Setup the "estlint" task to inherit its configuration settings from the wet-boew dependency's .eslintrc.json and .eslintrc.ci.json files.
• Updated src/theme.js to pass linting.
• Modified the "test" task to reference the "eslint" task.
• Removed an unused JSHint configuration file (.jshintrc).
• This is mostly a port of wet-boew/wet-boew#7456, along with some of wet-boew/wet-boew#7948's updates.
• Related to wet-boew/GCWeb#1312.

@nschonni @LaurentGoderre FYI.