[TACKLE-9] Stakeholder and stakeholder group duplication

[mrizzi]

While testing https://github.com/konveyor/tackle-ui/pull/136, I followed these steps (I tried to define the minimum set of steps to reproduce the issue):

1. select an application (not assessed yet) and asses
2. select one stakeholder and one stakeholder group
3. Save as draft button => this executes PATCH with this (partially reported) JSON payload:

   {
     applicationId: 28
     id: 418
     questionnaire: {,…}
     stakeholderGroups: [149]
     stakeholders: [129]
     status: "STARTED"
   }

4. Cancel button to go back to the application list
5. select the same application and asses => this executes the GET call to the /pathfinder/assessments/418 endpoint had this (partially reported) JSON response:

   {
     applicationId: 28
     id: 418
     questionnaire: {,…}
     stakeholderGroups: [149]
     stakeholders: [129]
     status: "STARTED"
   }

6. Save as draft button => this executes PATCH with this (partially reported) JSON payload:

   {
     applicationId: 28
     id: 418
     questionnaire: {,…}
     stakeholderGroups: [149]
     stakeholders: [129]
     status: "STARTED"
   }

7. Cancel button to go back to the application list
8. select the same application and asses => this executes the GET call to the /pathfinder/assessments/418 endpoint had this (partially reported) JSON response:

   {
     applicationId: 28
     id: 418
     questionnaire: {,…}
     stakeholderGroups: [149, 149]
     stakeholders: [129, 129]
     status: "STARTED"
   }

   and it seems like stakeholders and the stakeholderGroups doubled.

If I continue the Save as draft - Cancel loop, the stakeholders and the stakeholderGroups keep doubling every time.

It might be worth evaluating some DB constraints for ensuring unique (assessment_id, stakeholder_id) and (assessment_id, stakeholdergroup_id) non-deleted pairs.