Remove DBField casting when submitting via forms

[emteknetnz]

#11408 (comment) discovered that invalid DBField values are sometimes cast to a valid value before they are validated when submitted from a form. Instead what should happen is the value should not be cast and should instead cause a valid exception to be thrown

In that particular example the casting happening in DBBoolean::saveInto(). In was possible to get into this state by replacing the auto-scaffolded CheckboxField with a TextField instead, allows "this is not a boolean" to be the submitted value instead of 1 or 0.

Note that historically Silverstripe will often gracefully handle bad data rather than throw hard exceptions. This validation work is essentially the opposite way of handling bad data. Because of this, there may be some instances where this is too much of a change and we may need to soft things a little to reduce upgrade pain

Acceptance criteria

• Validation exceptions are thrown by DBFields when invalid data is POSTed into a form submission endpoint
• Validation errors are not thrown when valid values are entered into formfields (e.g. using a TextField with a DBInt should not result in validation errors saying "value must not be a string")