Keyboard a11y improvements

[dmfalke]

This PR addresses some issues with keyboard accessibility:

• General: Enable focus ring.
• PopoverButton: Restore focus to button when menu is closed.
• SwissArmyButton: Do not override outline style by default.
• CheckboxTree: Retain focus on internal node when toggling expansion state via keyboard.
• UserMenu (header): Make fully keyboard accessible (previously could not tab into it).
• Tooltip: Make fully keyboard accessible.
• Strategy: Make step edit button fully keyboard accessible.
• Strategy: Improve focus ring style for basket button in results table.
• Strategy: Make organism filter show/hide button fully keyboard accessible.

[dmfalke]

After merging this, I realized I should have used :focus-visible, instead of :focus. I will update this in a new PR.