Feature: Display the post a reply is referring to.

[TamaroWalter]

🔀 Purpose of this PR:

• Fixes a bug
• Updates for a new Moodle version
• Adds a new feature of functionality
• Improves or enhances existing features
• Refactoring: restructures code for better performance or maintainability
• Testing: add missing or improve existing tests
• Miscellaneous: code cleaning (without functional changes), documentation, configuration, ...

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📝 Description:

This PR implements #161.

What is the current situation?
When replying to a post, the user gets redirected to a new page and can't see the content of post the answer is referring to.

What is new?
Above the message block, a simplified version of the original post gets rendered. The post can be collapsed or expanded.

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📋 Checklist

Please confirm the following (check all that apply):

• I have phpunit and/or behat tests that cover my changes or additions.
• Code passes the code checker without errors and warnings.
• Code passes the moodle-ci/cd pipeline on all supported Moodle versions or the ones the plugin supports.
• Code does not have var_dump() or var_export or any other debugging statements (or commented out code) that
  should not appear on the productive branch.
• Code only uses language strings instead of hard-coded strings.
• If there are changes in javascript: I build new .min files with the grunt amd command.

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🔍 Related Issues

• Related to Feature request - The user should see the original message in the reply screen #161

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🧾📸🌐 Additional Information (like screenshots, documentation, links, etc.)

This is how the original post gets displayed:

collapsed	expanded

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