Fix issues with backup importing (#2092)

* Closes #2087 
* Use `python manage.py import_ctf` instead of a new Process to import backups from the Admin Panel. 
    * This avoids a number of issues with gevent and webserver forking/threading models. 
* Add `--delete_import_on_finish` to `python manage.py import_ctf`
* Fix issue where `field_entries` table could not be imported when moving between MySQL and MariaDB

manage.py

@@ -1,6 +1,8 @@
 import datetime
 import shutil
 
+from pathlib import Path
+
 from flask_migrate import MigrateCommand
 from flask_script import Manager
 
@@ -71,10 +73,14 @@ def export_ctf(path=None):
 
 
 @manager.command
-def import_ctf(path):
+def import_ctf(path, delete_import_on_finish=False):
     with app.app_context():
         import_ctf_util(path)
 
+    if delete_import_on_finish:
+        print(f"Deleting {path}")
+        Path(path).unlink()
+
 
 if __name__ == "__main__":
     manager.run()