Image manager: cleaned up style changes, dark mode support

Image manager: cleaned up style changes, dark mode support

- Updated tab handling to be smarter on initial tab selection, to first
  target non-hidden tab panels where they may be handled server-side.
- Extracted contained search box handling styles to _forms.scss, after
  merging with image-manager-specific styles since this is only usage of
  contained variant.
- Aligned focus handling on image manager UI elements.