Development: Fix postgres incompatibility for conversation user information query

[lennart-keller]

Checklist

General

• This is a small issue that I tested locally and was confirmed by another developer on a test server.
• Language: I followed the guidelines for inclusive, diversity-sensitive, and appreciative language.
• I chose a title conforming to the naming conventions for pull requests.

Server

• Important: I implemented the changes with a very good performance and prevented too many (unnecessary) database calls.

Motivation and Context

The query to retrieve user-related information about a conversation is incompatible with Postgres (see https://gist.github.com/TheZoker/66abc96cf0218d12ce888bda06a33dbc#file-e2e-postgres-error-log)

Description

The missing fields in the GROUP BY clause are added.

Steps for Testing

Prerequisites:

• 1 User
• PostgreSQL database

1. Log in to Artemis
2. Navigate to the messages tab of a course and confirm that the page loads.

Review Progress

Performance Review

• I (as a reviewer) confirm that the client changes (in particular related to REST calls and UI responsiveness) are implemented with a very good performance
• I (as a reviewer) confirm that the server changes (in particular related to database calls) are implemented with a very good performance

Code Review

• Code Review 1
• Code Review 2

Manual Tests

• Test 1
• Test 2

Test Coverage

Screenshots

No UI changes

[TheZoker]

E2E tests seem to be fixed 👍

[jakubriegel]

Code LGTM 👍🏻

Btw, there should be a way to validate @Query agains different dbs