codeintel: Small updates to code site-admin UI

Created by: efritz

• Add flash messages when items are created, updated, or deleted
• Have better "no items" messages in empty lists (see empty uploads list for good example)
  • Policy list (and maybe others) should have centered and padded text, a map icon, and a link to documentation in the form of a call-to-action
• Policy form should be in a single <Container />, not spread over multiple containers
• Create new policy button should be top right of page, not below filter
• Make sure delete buttons/links have tooltips
• Ensure git object filter input updates are throttled
• Add max height and scroll on git object filter so it doesn't jitter