Introduce linting to restrict line length to 120 chars

This adds an eslint rule to restrict line length to 120 chars.

This does not fix lines longer than this. These fixes will need to be
applied before merging

Author: tarrow

.eslintrc.js

@@ -16,7 +16,8 @@ module.exports = {
     'no-debugger': process.env.NODE_ENV === 'production' ? 'warn' : 'off',
     'vue/multi-word-component-names': 'warn',
     'vue/no-reserved-component-names': 'warn',
-    'vue/no-mutating-props': 'warn'
+    'vue/no-mutating-props': 'warn',
+    'max-len': ['error', {'code': 120}]
   },
   overrides: [
     {