Using submission_url from instances.db might block uploading forms

#### Software and hardware versions
Collect v1.24.1 and older versions

#### Problem description
submission_url from instances.db is used to upload filled forms, that mean if a user downloads a form with a wrong submission_url and fills some form he is not able to upload them even after downloading a fixed form.

#### Steps to reproduce the problem
1. Create and download a form that contains a wrong submission_url
2. Fill and save some instances
3. Realize that you added a wrong submission_url
4. Update your form and download it again
5. Try to upload already saved forms

#### Expected behavior
Forms should be uploaded using the updated submission_url.

#### Other information
It might be an issue especially if GS is used because if someone accidentally remove his spreadsheet and creates it again he is blocked.

Start by tracing how submission_url is read from instances.db and used during saved-form upload. Reproduce the described sequence with an outdated URL, then inspect the upload flow and related tests if present. Done means forms saved with an old URL can upload using the updated submission_url.

Written by the indexing model from the issue text.