Don't pop up keyboard when going back to search.
Prevent course list page from re-creating view models.
Some refactoring

o Remove MainViewModel from App
o Moved MainViewModel creation to MainPage instead
o Made PinnedCourses asynchronous and load on demand
o Removed await in course page code-behind, so that DataContext
  is set sooner, even though not everyting has been loaded yet
o Made course page not re-create view model every time it's opened