[EASI-4635] Discussions store methods and resolvers

[mynar7]

EASI-4635

Description

This PR implements the discussions API store methods and connects them to the GQL resolvers so that adding/retrieving discussions is functional instead of just using mock data.

How to test this change

• The Postman collection has endpoints created to create and reply to discussions.
• There are no automated tests yet.

PR Author Checklist

• I have provided a detailed description of the changes in this PR.
• I have provided clear instructions on how to test the changes in this PR.
• I have updated tests or written new tests as appropriate in this PR.

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[samoddball]

this looks really good, left a comment