Capx 88 | Required fields on mapper form

# Ready for review

* Adds required field validation to mappers
* Fixes a couple of bugs

Simply adding required to the form elements that are required causes errors in the form submission as it processes all fields as required when we only want a portion of the form. This is a problem with the way the form is generated to begin with but it is easier to work around it. Validation for required fields has to be on the validation hook. This is not a problem. The mapper form uses a theme_wrapper that replaces a field's label with the required asterix (*) with a preg_replace. This is kind of yucky and am open to suggestions on a better fix.
The reason it's not using that is because all entity fields and properties are exposed on the page, but only the selected entity is displayed. Using #required doesn't work with this way of rendering the form. Ideally, the form should be re-generated when a different entity (and bundle) is selected and then #required would work fine.