Use a stub PageAccess implementation in the pure-web environment

When running in a web page (as opposed to extension), use
a stub PageAccess implementation and avoid checks for page access
null-ness.
Improve keyboard navigation of item list and details view

 * Add Alt+Shift+P hotkey for password manager in Firefox addon

 * Enable hovering items in the item list using up/down arrow
   keys

 * Enable selecting the hovered item in the item list by pressing
   Enter

 * Pressing Backspace in the item detail view switches back
   to the main item list

 * Pressing 'a' in the item detail view autofills the current
   page using the active item's fields