[Forms] Add a custom success message for forms

[tudoramariei]

As a user, I want to give users a custom message after they submit a form.
The success message of all the forms can only be customized by modifying the form.event.sent language key.

Ideally, the key should remain, but the custom message should override it.
If it's not feasible to have both options, the On Submit Message section should be mandatory.