Communication: Allow file uploads in messages

[pzdr7]

Checklist

General

• I tested all changes and their related features with all corresponding user types on a test server.
• 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) and too complex database calls.
• I strictly followed the principle of data economy for all database calls.
• I strictly followed the server coding and design guidelines.
• 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 (e.g. using paging).
• 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

In the communication feature, it's currently not possible to upload files or link images, as the message will just remain blank.

Description

• Added the file upload to the markdown editor in the communication view
• Added endpoints for storing and retrieving markdown files in communication
• Allows users to embed images in their messages

Steps for Testing

Prerequisites:

• 1 Student
• 1 Course with communication enabled

1. Navigate to a channel, private message, or group chat.
2. Upload files into the editor:

• Click the file upload footer and select an image file. Verify that a link gets inserted and that when you send the message, the image is embedded. When you click on the image, it should expand.
• Select a file that is larger than 5 MiB, but smaller than 20 MiB. There should be an error message.

3. Check that the other reference types (e.g. user, channel) are still displayed correctly.
4. Check that you can link to external images, e.g. by inserting ![ase](https://avatars.githubusercontent.com/u/11064260?s=200&v=4)

Testserver States

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Review Progress

Code Review

• Code Review 1
• Code Review 2

Manual Tests

• Test 1
• Test 2

Test Coverage

Screenshots

Screencast.from.24.10.2024.20.23.45.webm

Summary by CodeRabbit

Release Notes

• New Features

  • Introduced constants for file size and submission limits to enhance configuration consistency.
  • Added methods for managing markdown file uploads and retrievals specific to course conversations.
  • Enhanced the markdown editor to support file uploads for both images and PDFs.
  • Implemented new error messages for missing image references in the markdown editor.
  • Updated the component to utilize reactive state management for posting content parts.
  • Improved file upload functionality with dynamic file type acceptance based on allowed extensions.

• Bug Fixes

  • Improved error handling for oversized file uploads in the file resource.

• Documentation

  • Updated internationalization files to include new messages for image reference errors.

• Tests

  • Expanded test coverage for file upload scenarios and component behavior adjustments.

[dmytropolityka]

manual test on ts1, also code

[github-actions]

⚠️ Unable to deploy to test servers ⚠️

Testserver "artemis-test5.artemis.cit.tum.de" is already in use by PR #9611.

[github-actions]

⚠️ Unable to deploy to test servers ⚠️

Testserver "artemis-test1.artemis.cit.tum.de" is already in use by PR #9395.

[Fallthunderclap]

Tested on test-server-2
Followed steps for testing and haven't encountered any issues.