Require const declarations for variables that are never reassigned after
being declared. If a variable is never reassigned, using the const
declaration is better. const declaration tells readers, “this variable
is never reassigned,” reducing cognitive load and improving
maintainability.
Validation is not enforced on the image output step
when selecting and then deselecting an image type. This
is because the only validation which is set is `Required`.
However, after deselecting an image type the input becomes
a key-value object with all the values set to false. This
commit adds a custom validator to check that at least one
value in the object is set to true. Fixes#575
