Fix #4009: reorder the registration of routes (#4017)

Per the mux docs, "Routes are tested in the order they were added to
the router. If two routes match, the first one wins."

This means that `checks.RegisterRoutes()` needs to be called after
everything else defining a route in /api/checks/ as it otherwise
matches every path under /api/checks/.

webapp/web/main.go

@@ -18,15 +18,24 @@ import (
 )
 
 func init() {
-	// webapp.RegisterRoutes has a catch-all, so needs to go last.
-	api.RegisterRoutes()
+	// API routes:
+
+	// /api/checks/ routes:
 	azure.RegisterRoutes()
-	checks.RegisterRoutes()
 	ghactions.RegisterRoutes()
+	// checks.RegisterRoutes has a catch-all for /api/checks/, so needs to go last.
+	checks.RegisterRoutes()
+
+	// The rest of /api/:
+	api.RegisterRoutes()
 	query.RegisterRoutes()
 	receiver.RegisterRoutes()
 	screenshot.RegisterRoutes()
 	taskcluster.RegisterRoutes()
+
+	// The actual Web App:
+
+	// webapp.RegisterRoutes has a catch-all, so needs to go last.
 	webapp.RegisterRoutes()
 }