Fix page redirects (#620)

* Fix broken redirects

A few of these were wrong since they assumed the target path is
relative to the root of the course (the path is relative to the page
being redirected).

* Sort redirects

book.toml

@@ -44,21 +44,21 @@ editable = true
 [output.html.redirect]
 # Redirects in the form of "old-path" = "new-path", where the new path
 # is relative to the old path.
-"structure.html" = "running-the-course/course-structure.html"
+"exercises/day-4/afternoon.html" = "../android/morning.html"
+"exercises/day-4/android.html" = "../android/morning.html"
+"exercises/day-4/dining-philosophers.html" = "../concurrency/dining-philosophers.html"
+"exercises/day-4/elevator.html" = "../concurrency/elevator.html"
+"exercises/day-4/link-checker.html" = "../concurrency/link-checker.html"
+"exercises/day-4/morning.html" = "../concurrency/morning.html"
+"exercises/day-4/solutions-morning.html" = "../concurrency/solutions-morning.html"
+"generics/closures.html" = "../traits/closures.html"
+"generics/impl-trait.html" = "../traits/impl-trait.html"
+"generics/trait-bounds.html" = "../traits/trait-bounds.html"
 "running-the-course/day-4.html" = "course-structure.html"
-"exercises/day-4/afternoon.html" = "exercises/day-4/android.html"
+"structure.html" = "running-the-course/course-structure.html"
 "unsafe/unsafe-functions.html" = "calling-unsafe-functions.html"
-"generics/closures.html" = "traits/closures.html"
-"generics/impl-trait.html" = "traits/impl-trait.html"
-"generics/trait-bounds.html" = "traits/trait-bounds.html"
 "welcome-bare-metal.html" = "bare-metal.html"
 "welcome-day-4.html" = "concurrency.html"
-"exercises/day-4/android.html" = "../android/morning.html"
-"exercises/day-4/morning.html" = "../concurrency/morning.html"
-"exercises/day-4/solutions-morning.html" = "../concurrency/solutions-morning.html"
-"exercises/day-4/dining-philosophers.html" = "../concurrency/dining-philosophers.html"
-"exercises/day-4/link-checker.html" = "../concurrency/link-checker.html"
-"exercises/day-4/elevator.html" = "../concurrency/elevator.html"
 
 [output.exerciser]
 output-directory = "comprehensive-rust-exercises"