[124] Redesign register account page

[Mar976]

https://trello.com/c/NPwFL0ao/124-create-your-account-page-enhancement-3

What we did

• styling of inputs
• adding texts to describe inputs
• disabling button when either input is invalid
• input validation
  • name: max length, not empty
  • username: regex, max length, not empty
• accessibility fixes