Move darkstorm_backend to just backend

Added MongoDB instances of DB tables
Updated some DB interfaces
Added logging to count cleanup

internal/backend/app.go

@@ -0,0 +1,16 @@
+package backend
+
+import "net/http"
+
+// An application interface. Both LogTable and CrashTable are optional, if they return nil then requests will be forbidden.
+type App interface {
+	AppID() string
+	CountTable() CountTable
+	CrashTable() CrashTable
+}
+
+type ExtendedApp interface {
+	// Extension is called for any calls to /{appID}/
+	// Alternatively, use Backend.HandleFunc for more customizability
+	Extension(http.ResponseWriter, *http.Request)
+}