Select projects using the arrow keys in the projects dropdown

This was a request/complaint from @SavePointSam on twitter:

If you type in the search in the main nav, you can use the arrow keys to select an option in the list.

In the project drop down menu, the search doesn’t support using the arrow keys to select and navigate with enter.

I don’t use the project menu anymore because of it :(

In many other places in our UI, (global search, fuzzy search, select2 dropdowns, etc) we allow the user to use the ↑ and ↓ arrows to highlight an item, and Enter to select it. It would be useful, and consistent, if we followed this same pattern in the Projects/Groups dropdowns in the main nav menu.