Iris: Fix two UX issues

[Hialus]

Checklist

General

• I tested all changes and their related features with all corresponding user types 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.
• I followed the coding and design guidelines.
• I added multiple integration tests (Spring) related to the features (with a high test coverage).
• I added pre-authorization annotations according to the guidelines and checked the course groups for all new REST Calls (security).
• I documented the Java code using JavaDoc style.

Client

• Important: I implemented the changes with a very good performance, prevented too many (unnecessary) REST calls and made sure the UI is responsive, even with large data.
• I followed the coding and design guidelines.
• Following the theming guidelines, I specified colors only in the theming variable files and checked that the changes look consistent in both the light and the dark theme.
• I added multiple integration tests (Jest) related to the features (with a high test coverage), while following the test guidelines.
• I added authorities to all new routes and checked the course groups for displaying navigation elements (links, buttons).
• I documented the TypeScript code using JSDoc style.
• I added multiple screenshots/screencasts of my UI changes.
• I translated all newly inserted strings into English and German.

Motivation and Context

There were three minor issues with the client code of Iris:

1. The resend feature did not work
2. If certain errors happened, it was impossible to start a new session
3. The tests did not pass

Description

This PR fixes all three issues:

1. Resend now works again in all relevant cases
2. You can now always start a new session
3. The tests pass

Steps for Testing

(Don't be scared by the length, they are just very detailed 😉)

Prerequisites:

• 1 Instructor

1. Log in to Artemis
2. Navigate to exercise 1 in course 1 (student view)
3. Start the exercise (if not already done previously)
4. Wait for it to be started
5. Open Iris by clicking on the speech bubble in the bottom right
6. Send a message, wait for a response -> works
7. Now close Iris, click the "View" button and then click the "Iris" button
8. Change the first {{#system~}} to {{system~}}
9. Go back to the student view
10. Send a message -> Error occurs
11. You can see the resend symbol -> click it -> error again obviously
12. You can start a new session by clicking on the trash can
13. Send a message -> does not work
14. Important: Change the {{system~}} to {{#system~}}!
15. Resend the message -> works

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

[github-actions]

❌ Unable to deploy to test servers ❌

Testserver "artemis-test9.artemis.cit.tum.de" is already in use by PR #6931.

[RY997]

The code overall looks good to me, leaving two small comments.
And will test it after ts9 is released.