Refactor RequiredField validator

[maxime-rainville]

The current behaviour for RequiredFields is weird. It runs the individual validation for each field which seems like mixed concerns.

It also happily accepts a white space has a non-empty string.

[GuySartorelli]

IMO this should be split into two separate validators - one for the required fields validation and another entirely separate validator for calling validate() on each field.
The latter of these could by default be included in scaffolded composite validators (e.g. on CMS edit forms).
We've implemented that.

As for the whitespace we should make it not accept white-space as a "value" (i.e. if the value is only whitespace for a required field, throw a validation error) - but perhaps add a configuration option to allow it, in case someone has some valid use-case for whitespace-only values.

[GuySartorelli]

Closing as a duplicate of #10908