Fix exception when adding a video to a post

[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 fixes #248.

When a user created a new discussion and added a video to the first post, the moodleoverflow crashed. When a user tried to edit a post by adding a video, the video would not play.

What was the problem?
Both situation were based on the same problem. THe post-class function that moves files from the form into a permanent file area (file_save_draft_area_files) used a $draftid for
module descriptions (introeditor) and not for post messages. Due to that, files like videos were not uploaded into the draft area and an error ocurred.
A new discussion in particular crashed in this scenario because of the way a post is created. A discussion is created before the post and because of that the discussion-object references
0 for the firstpost. Then a post is created (which needs a valid discussion id to be created): the post is inserted into the database, the files are uploaded in the file area, the post gets updated in the database with the new files and then the post-id is returned to the discussion, which updates the post reference.
Due to the draft-id error, the post was in the database but not rightly updated and the discussion could not reference it because the firstpost still had 0 as value. The render functionality then crashed because it did not found the post with id 0

What is the solution?

The solution is quite simple. By building the $draftid for message forms, moodle can upload and show the videos. The post and the discussion get updated correctly and everything works as it should.

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📋 Checklist

Please confirm the following (check all that apply):

• 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.

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🔍 Related Issues

• Related to Bug - adding video to message #248