Dependencies cleanup

Summary

• Removed and replaced some dependencies
• Added some new validation rules

Complete changelog

Added

• Validation rules
  • New borrowing
    • The expected return date can be at most one month away.
  • Delete user
    • The user must not be involved in an ongoing borrowing.

Changed

• Environment
  • Moved public folder content to resources
  • Replaced Bootstrap with Bulma
  • Removed jQuery
  • Replaced flag-icon-css with a custom lighter build
  • Removed @babel/polyfill