Create Material-UI Navbar

[flurmbo]

Create a Navbar using Material-UI componenets to appear at the top of each page. This should probably use the AppBar component. We should be able to navigate between the check-in page, the retail page, the dashboards, and the leaderboard.

The navbar should handle different screen sizes well.

The leaderboard link should, link to the current year's leaderboard. but in the future we might want it to open a drop down containing the different years of leaderboards available. That depends on what the people at Green streets think is best.