Fixed header and footer on confirmations

While one goal for snap developers _should be_ to design their confirmation screens so that key information is displayed within & above the fold—there are some scenarios where the content will exceed our screen real-estate.

To address this, the header & footer should be fixed by the top / bottom and i.e. only the middle _content container_ is scrollable. So for cases where the middle content is higher than its container -> the confirmation's CTA-button will be _disabled_ until the user has scrolled / clicked to the bottom (and reviewed all information).

Design here: https://www.figma.com/file/Wlwk0cTx6PQkNhiX7vN13e/Snaps-Platform-v1?type=design&node-id=412%3A19223&mode=design&t=IbsGxB2xZNdc9LNi-1

Start by reviewing the linked Figma design and locating the confirmation-screen implementation in the MetaMask extension. Check how the confirmation header, middle content container, footer, and CTA are currently rendered. Done means the header and footer remain fixed, only the middle content scrolls, and the CTA stays disabled until the required content has been reviewed.

Written by the indexing model from the issue text.