Fix custom form activation to solve double slot validation issue - [ENG 117]

[varunshankar]

Proposed changes:

• Refactor form activation to solve double slot validation.
• Add tests for the change. Fixes https://rasahq.atlassian.net/browse/ENG-117

Status (please check what you already did):

• [x] added some tests for the functionality
• [ ] updated the documentation
• [x] updated the changelog (please check changelog for instructions)
• [x] reformat files using black (please check Readme for instructions)

[github-actions[bot]]

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