Update to actively supported focus-trap library

[brentswisher]

The problem
We are using @ithaka/focus-trap which is an out of date fork of a no longer maintained package to provide focus trapping for accessibility purposes. This package calls document directly in it's initialization which conflicts with the Gatsby SSR of pharos-site, requiring a clunky workaround on all pages for components which use it.

The solution
Update to a maintained library or write our own focus-trap inside of Pharos which is compatible with Gatsby SSR.

Additional information

• https://github.com/focus-trap/focus-trap looks to be well maintained
• Use the standard popover API where appropriate #755 and Leverage native dialog element for Modal #347 may impact this if the focus trap is no longer needed