Customize gas modal should use state.metamask.send instead of component state

[danjm]

Customize gas modal uses its own local component state. Meanwhile, the state is manages is almost all on state.metamask.send. Seems like a potential source of bugs if customize gas modal local state and state.metamask.send get out of sync somehow.

Lets refactor the customize gas modal to use redux state.

(Estimating this as a 3 only because of unknown side effects of the change that we might have to correct.)

[danjm]

This will be covered by: #3725