Add ESLint integration and fix linting issues

[ibrahimhabibeg]

• Integrate eslint with the web app to easily detect errors and follow coding standards.
• Add npm script to quickly check for errors and warnings
• Integrate with react recommended rules
• Eslint is integrated with most text editors and IDE (ex: VS code) for fast error detection and fixing
• Fix 26 errors and 5 warnings detected by eslint