Refactor signin form to prepare signup form integration.
Enable switching between signin and signup.

+ Submit for signup not implemented yet.
Fix typo.
Prepare submitting signup form data.

+ TODO: Extract validation for password confirmation.
Change wording to signin.
Implement client-side signup request.

+ API has not been implemented yet.
Fix test description.
Avoid storing states in the DOM.
Add test for hiding signup popup.
Merge branch 'feature/signUpFormIntegration' into prototype