Add a responsive modal option

[theGreener]

Is your feature request related to a problem? Please describe.
On large screen devices modals don't make full use of the available space for them, this results in labels for form fields overflowing onto new lines making it difficult to read the label

Describe the solution you'd like
Modals should be responsive to the amount of space available

Describe alternatives you've considered
We're already using the largest modal size (modal--large), an alternative approach would be to provide modal sizes above large

Additional context
An example modal where the labels are squeezed

[Stanton]

Responsive modal is the easy bit, the illustrated problem mostly requires a responsive form layout which is a significantly bigger challenge...

[Stanton]

@theGreener Any chance you can DM me the contents of that modal as test data?