Add eslint-config-problems and fix errors

Long overdue!

• Add eslint-config-problems and resolve offences (var, etc.)
• Enforce more import/* rules (no extension, no unresolved, etc.)
• Remove duplicate imports, removes .js extensions, sort imports, install missing dependencies, etc.
• Replace react-time-ago, which requires configuring en locale and importing unresolved package (or installing a second package), with react-timeago.