import: Fix ordering of subdomain availability check.

When you're importing with --destroy-rebuild-database, we need to
check subdomain availability after we've cleared out the database;
otherwise, trying to reuse the same subdomain doesn't work.

zerver/management/commands/import.py

@@ -65,7 +65,6 @@ import a database dump from one or more JSON files."""
         if subdomain is None:
             print("Enter subdomain!")
             exit(1)
-        check_subdomain_available(subdomain)
 
         if options["destroy_rebuild_database"]:
             print("Rebuilding the database!")
@@ -75,6 +74,8 @@ import a database dump from one or more JSON files."""
             for model in models_to_import:
                 self.new_instance_check(model)
 
+        check_subdomain_available(subdomain)
+
         for path in options['export_files']:
             if not os.path.exists(path):
                 print("Directory not found: '%s'" % (path,))