feat: Add Form validation

[victorgcramos]

This PR adds validation to all forms on politeia, including error messages, and form submission control.

• Login Form validation
• Signup form validation
• User key update form validation
• user password change/reset form validation
• username change form validation
• email verification form validation

[victorgcramos]

@tiagoalvesdulce This will be added once the user layer gets implemented. Sounds good? We need that to get the password min and max lengths, so I can replace MIN_PASSWORD_LENGTH with the policy values as well