[#25] Fix delete survey APIs

[roberlander2]

Description

This PR fixes a bug that occurs when trying to delete surveys and it adds a unique partial index on the calendar_event_id field to the surveys collection to prevent duplicate event surveys.

Resolves #25

Review Time Estimate

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• [x] Immediately
• [ ] Within a week
• [ ] When possible

Type of changes

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• [x] Bug fix (non-breaking change which fixes an issue).
• [ ] New feature (non-breaking change which adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [ ] Other (any another change that does not fall in one of the above categories.)

Checklist:

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• [x] I have signed the Rokwire Contributor License Agreement (CLA). (Any contributor who is not an employee of the University of Illinois whose official duties include contributing to the Rokwire software, or who is not paid by the Rokwire project, needs to sign the CLA before their contribution can be accepted.)
• [x] I have updated the CHANGELOG.
• [x] I have read the Contributor Guidelines.
• [x] I have performed a self-review of my own code.
• [x] I have commented my code, particularly in hard-to-understand areas.
• [ ] My change requires updating the documentation.
• [ ] I have made necessary changes to the documentation.
• [ ] I have added tests related to my changes.
• [x] My changes generate no new warnings.
• [x] New and existing unit tests pass locally with my changes.
• [x] Any dependent changes have been merged and published in downstream modules.

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