Add AJAX form validation

[bennothommo]

Fixes #628.

This PR adds the Form validation feature to the Snowboard extras that is already documented but was left missing in the original Snowboard implementation.

• Docs: https://wintercms.com/docs/snowboard/extras#ajax-validation
• Workshop Theme examples: wintercms/wn-workshop-theme@431e761

[what-the-diff]

• Removed the display: none; from [data-request][data-request-validate] [data-validate-for],
  [data-request][data-request-validate] [data. This was causing validation errors to not show up on forms when using AJAX requests.
• Added a new file called FormValidation which handles showing and hiding of error messages for form fields that are validated via an AJAX request, as well as adding them back into Snowboard's plugin list so they can be used in themes/plugins etc...