[Cashtab] Improvements to modal component

Summary: patch css typo, add an overlay to prevent clicking outside of the modal, fix button positioning, do not use transparent background for cancel button as validation errors may be viewed through it, add support for disabling the ok button

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15909