Add system header and footer

Large organizations that store confidential information like trade secrets in their GitLab instance frequently want to have a permanently present header and footer to make sure that all staff are always aware of this.

Design

• Add new system header and footer option to the instance appearance configurations
  • system header
  • system footer
  • background color (note: make sure this is in sentence case)
  • text color (note: make sure this is in sentence case)
• Should follow same styling as flash banner messages
• The header, or footer, or both can be set in any combination.
• If the header is enabled, the header should always (sticky) be visible in the viewport and above the top nav
• If the footer is enabled, the footer should always (Sticky) be visible at the bottom of the viewport
• Banners are non-dismissable
• There should be a template text stating: State your message to activate

Admin	User