As a user I want a var created for modal width

[blazebarsamian]

I'm submitting a ...

• bug report
• feature request
• support request

What is the current behavior?

Can't change modal width without overwritting the styles. Would be nice to be able to change the width by updating a var in the settings file.

What needs to happen first before users can update modal widths in the harmonium settings file?

• New var is added to harmonium-settings for the modal width
• New var is implemented in the Modals style sheet

What is the expected behavior?

• When I update the width var in the settings file
• Then the width of the modal changes to reflect the new size.

For feature requests: why should the behavior change?
To make it easier for designers to update the modals width without having to overwrite styles.

Other information (we love stack traces!)

• Make sure we add the new var to the props and vars tables (As a user, I want the vars & props for the Modal component documented on the site #290)