Add simple_form

- Add a Bulgarian locale file for simple_form
- Use simple_form for the registration page
- Add a custom component for Abide client-side validations
Update and lock foundation-rails to 5.3.3

We’ll vendor the abide plugin to make a change in it.
Remove the switch component, as there is no such file to require in Foundation 5.3.3.
Vendor the abide plugin from Foundation 5.3.3
Add a config setting for the abide error class name
Show either client- or server-side errors, not both

- Use abide-error and abide-message classes for Abide
- Don’t show Abide errors on fields w/ server errors
- Hide server validations on change. This will allow client-side validations to
  show up.
First and last name are required user fields