Improve admin keyboard accessibility

Currently there is no keyboard accessibility in the Perch admin (v3).

Further more it seems like focus outline has been disabled in CSS (below) without any substitute.

:focus {
    outline: 0;
}

I'm not doing this out of snark but… https://www.outlinenone.com/. As a keyboard user this is frustrating and a quick fix would be to remove this line of CSS.

Further improvements would be to use some "skip links" to help with navigating around the admin. Just to add I have no motor impairments but prefer navigating with the keyboard as it's faster.