Users with no current enrollments or groups should still have link to /courses, and /courses should not be an empty page

**Don't merge this yet. I want to try using this PR as a discussion tool to see if this is the right approach.**

If a user with no current course enrollments or group memberships logs into Canvas, the Courses dropdown in the top nav is replaced with a "Home" link to the dashboard. This is not a great experience:

* The user may have past enrollments (e.g. a sessional instructor) and has no way of getting to them without knowing the /courses URL.
* The "Home" link is redundant since the logo, directly to the left of Home, also links to the Dashboard.

We discussed fixing this in custom JS, but I believe that this behaviour is wrong and should be fixed in Canvas, so I took a stab at doing so.

Fixing the nav was easy (app/views/layouts/application.html.erb) -- just change the link from dashboard_path to courses_path.

A secondary issue is that if the user has absolutely no enrollments or groups at all, current or past, then the /courses page is completely blank. Also not a good user experience so I fixed that as well.

I'm going to ask in IRC if this is something Instructure would consider pulling in. If not, we can either merge this in to our fork anyway, or look at a JS fix instead.

Thoughts?