Add filter to block honey pot field entries - dev

[nickpagz]

Changes proposed in this Pull Request

• The Events Manager plugin doesn't have a great way to block spam entries on booking forms.
• This PR adds a function to stop bots from submitting the form.
• A filter is added to check for a custom form field with the slug phone_hp, and block registration for the event if it is not empty.
• CSS is added to hide the field on the front-end.
• The booking form field must have the slug phone_hp, and must be a non-required text field.

Mentions #187