Admin UI: Unify focus styling across links, tabs, buttons, etc

Focus styles are useful for users who use the keyboard to navigate through a site. In the current UI, there are focus styles for almost everything, but they aren't consistent (or even good, in some places). See below, there are 3 different kinds of style for focus on just this one section of page. And calypso form components use yet another style.



The first two in the first screenshot are OK, but the last ("Weeks"), is not different enough from the currently-selected item; additionally there is no focus indication when landing on the currently selected item.

The setting screens are otherwise good, the form styling from calypso is carried over.

Start by comparing focus behavior in the dashboard and settings screens shown in the issue, including the Calypso form styling mentioned there. The work is done when links, tabs, buttons, and selected items have consistent, visible keyboard-focus indications without confusing focus with selection.

Written by the indexing model from the issue text.