Add/handle require username. Fix #21

List of tasks to review:

- [x] Signup with existent email should fail
- [x] Signup with existent username should fail
- [x] Signin with username should work
- [x] Signin with email should work
- [x] Signup with username+email should work
- [x] Fix borders styles
- [x] Assign a new icon for the email input (different from the identity username) cc @gravityonmars 
- [x] Review all signin/reset/signup html forms. Make sure there are no conflicts with previous features like `usernameStyle = 'username'`. This should now hide/show the username input and disable email when enabled.