API: Return URL to submission

[zepich]

The idea is to return the URL to the submission in the API response after the submission is verified.

The system could use this URL to store the information in the backend or send the link to the recipient of a submission.

Example:

1. A frontend user fills out a form, clicks the mosparo checkbox, and submits the form
2. The system (for example, WordPress website) verifies the data with the mosparo verification API
3. mosparo returns that the data contains no spam. Additionally, mosparo returns the URL to the stored submission.
4. The WordPress plugin emails the submission to the website administrator. The URL to the submission was added to the email to the administrator.
5. The administrator sees that there is spam in the submission. The administrator can access the submission in the mosparo backend with a simple click on the link.