Add start time to scheduled reports

[madelondohmen]

Changes

This PR adds a start time to the scheduled reports.
When scheduling a report, a user can now choose a start time as well. (Default is the current time, UTC)

Issue link

Closes #3738

Demo

QA notes

• Schedule a report
• Change the scheduled date and time
• Check if everything is as expected on the Scheduled Reports page

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Code Checklist

• All the commits in this PR are properly PGP-signed and verified.
• This PR only contains functionality relevant to the issue.
• I have written unit tests for the changes or fixes I made.
• I have checked the documentation and made changes where necessary.
• I have performed a self-review of my code and refactored it to the best of my abilities.

• Tickets have been created for newly discovered issues.
• For any non-trivial functionality, I have added integration and/or end-to-end tests.
• I have informed others of any required .env changes files if required and changed the .env-dist accordingly.
• I have included comments in the code to elaborate on what is not self-evident from the code itself, including references to issues and discussions online, or implicit behavior of an interface.

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Checklist for code reviewers:

Copy-paste the checklist from the docs/source/templates folder into your comment.

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Checklist for QA:

Copy-paste the checklist from the docs/source/templates folder into your comment.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
81.4% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud