Improve User Settings email validation and error flow

[sdirosa]

This PR solves #1281

Changes

• In the users api:
  • added invalid_email error code when email validation fails
  • added unit test to verify whether sending an invalid email returns the new error code
• Improved a few things in the frontend for the settings view
  • In the template, the email field's type attribute is now set to email, instead of text
  • The form will remain in editing mode when receiving errors from remote
  • The invalid_email error code will result in a message being shown under the email field

[knolleary]

Thanks @sdirosa