fix(GlFormSelect): fix inconsistent select disabled state

What does this MR do?

A tiny MVC to handle #1017 (closed) where we align the select disabled state with the rest of the form inputs. Ideally we could share the styling between gl-form-input and the select component but this creates some strange bugs, perhaps we could abstract the disabled state to a shared mixin in a follow-up?

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Closes #1017 (closed)