Merge branch '6825-existing_db_gitignore' into 'develop'

Add install/existing_db.sql to .gitignore

Closes #6825

See merge request ispconfig/ispconfig3!1981

Author: Till Brehm

.gitignore

@@ -64,3 +64,4 @@ Temporary Items
 /server/lib/config.inc.php
 /server/lib/config.inc.local.php
 /interface/lib/config.inc.local.php
+/install/existing_db.sql