Implement front-end form validation

[hugopeek]

The current FormIt validation is sufficient, but things would greatly improve if there's an additional validation layer in front of that.

A few benefits:

• Immediate feedback, instead of after a refresh
• Refresh comes with the problem of loosing focus of your form (unless you use an anchor in the form action, but that has other side effects)
• You can use specific input validation, such as formatting for phone numbers or credit card details
• It might solve the issue of validating fields that are only required based on another input value (such as alternative address) or inputs inside collapsible sections. The backend issue is here: hugopeek/romanesco-patterns#32
• It might be easier to add validation parameters through the CBs this way