[BD-38][INF-544][BB-6682] feat: implement reverse_order parameter

[0x29a]

Description

• Added reverse_order query parameter to the threads endpoint.
• Modified merge_response_content to support reversed content order.

Testing instructions

1. Open Discussion for some course.
2. Create a discussion, add two responses, add comments for both of them.
3. Specify the thread ID (you can find it in the address bar) and course ID, and run the following (httpie used):

   http http://127.0.0.1:4567/api/v1/threads/<YOUR_THREAD_ID> "X-EDX-API-KEY: forumapikey" course_id==<YOUR_COURSE_ID> with_responses==True user_id==4 mark_as_read==False recursive==True resp_skip==0 reverse_order==false

   You should see a thread with comments ordered by the creation date.

4. Run the command from the previous point, but change reverse_order==false to reverse_order==true. You should see threads and comments in the reversed order.
5. You can specify resp_limit==1 to verify that the reverse ordering works with pagination.

[openedx-webhooks]

Thanks for the pull request, @0x29a!

When this pull request is ready, tag your edX technical lead.

[asadazam93]

@0x29a can you please rebase this?

[0x29a]

@asadazam93, done.

[openedx-webhooks]

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