Allow theme config to set server-side required

[zerocrates]

Makes "required" fields more robust, and fixes usage of elements that
Laminas defaults as required (here we're defaulting to not-required).

(fix #1779)

[kimisgold]

This works as expected. I tested using a date element.

1. With no "required" attribute defined, the date element defaulted to not required.
2. Setting attributes.required = false remained not required.
3. Setting attributes.required = true made the element required, and I could not submit the form until configuring this field.

The one small bump was setting attributes.required = "false" with quotes around the value made it set required="required" on the element, but that may not be an issue with proper documentation or assumed familiarity with setting boolean values in Laminas.

[zerocrates]

Hmm I guess that's just how the ini parser works. Not sure we can really change that. When you wrote it that way it did the whole thing, with the asterisk and all, right?

[kimisgold]

When you wrote it that way it did the whole thing, with the asterisk and all, right?

Yup! Presentation matched functionality.