Add import export commands to manage.py (#1723)

* Add `import_ctf` and `export_ctf` commands to `manage.py`
* Deprecate `import.py` and `export.py`
* Works on #1629

import.py

@@ -8,4 +8,7 @@ import sys
 
 app = create_app()
 with app.app_context():
+    print(
+        "This file will be deleted in CTFd v4.0. Switch to using `python manage.py import_ctf`"
+    )
     import_ctf(sys.argv[1])