EuiExpressionButton is not keyboard accessible

[nreese]

Tabbing does not open popovers in EuiExpressionButton.

[chandlerprall]

They are tabbable, but don't have any styles to indicate focus.

@cchaos what do you think about something like

.euiExpressionButton:focus {
  border-bottom: $euiBorderThick;
}

[cchaos]

It could probably just use the same styles as the active state since it's just slightly hard to tell the difference when it goes from dashed to solid with the same color.