Improve fix linter for CI

There are some changes we should do to improve our code lint.

[ ] do not fix linter errors or warnings
[ ] do not allow warnings
[ ] change no-console/no-debugger rules
- add debug (https://www.npmjs.com/package/debug) module and use it instead of console.log
[ ] prefer const when not reassign
[ ] lint tests/ folder as well
[ ] fix new issues fond

This can be achieved by adding or changing:

// in package.json
"lint": "vue-cli-service lint --no-fix --max-warnings 0 src/ backend/ tests/"

// in .eslintrc.js
rules: {
  'no-console': 'warn',
  'no-debugger': 'warn',
  'prefer-const': 'error',
},

paulodiovani / hacktoberrank