Update header/footer for file viewer

[dbranchini]

Originally part of a redesign of the file viewer - See #1849 and https://www.figma.com/proto/LVP13xZ0HhrxAPHCnoMVW1/Embedded-File-Viewer?page-id=0%3A1&type=design&node-id=316-1113&viewport=-2703%2C-153%2C0.5&t=drUPK0C4xcXsgdsS-1&scaling=min-zoom&starting-point-node-id=316%3A1113&mode=design for reference.

In order to solve accessibility issues, we decided to make #1849 focused on the files/folders UI. We split out the header and footer work into this ticket. The larger question to answer - should this ticket include all viewers (except the Mirador book viewer and the media viewer since that's being handled separately)?