Improve form validation

[Pashaminkovsky]

Error messages on forms (e.g. if a required field is missing) currently appear at the top of the form. If forms are long and the top is not visible, it is not obvious to users why a form isn't submitting.

Can we make the error more visible to end users either by disabling the submit button until all required fields are populated, with a tooltip saying "Required field(s) are empty." Open to other suggestions as well.