Links behind "Quick! Join Keybase" popup can be activated via tab + enter

Granted, this is a minor issue but still...

When you click on "Whatever, I Accept It" to create a new account from an invitation, a "popup" window appears. Even though everything behind the popup is grayed out, the links there can be activated by pressing the tab key a few times, something which is not uncommon when filling in a form.

Pressing the tab key should make the focus jump only to different controls on the popup window.

Start by reproducing the invitation account-creation flow after clicking "Whatever, I Accept It," then inspect the resulting "Quick! Join Keybase" popup and its focus behavior. Done means repeated Tab presses keep focus within the popup controls and do not activate links behind the grayed-out window.

Written by the indexing model from the issue text.