Give Service Desk more prominence in the UI

What does this MR do?

Remaining Todos:

• Only display nav item when service desk is enabled for project
• Only allow navigation when service desk is enabled for project
• Fix highlighting of nav item when service desk page is active
• Pre-populate search bar with support bot as user
• Don't clear author (support bot) on clear search button
• Render actual incoming email address in empty state
• When issues list isn't empty, display user callout
• Improve styles and markup for service desk info states
• Refactor/Clean up JS
• Add spacing before "Turn on Service Desk" button
• Move nav item above Milestones
• Add bottom border to non-empty-state graphic
• Write feature tests
• Write JS unit tests
• Write documentation

Screenshots

What are the relevant issue numbers?

Closes #3049 (closed)