refactor: generalized discussions

[samuelstroschein]

Follow up of #3189 with the insight of #3191.

• general discussion table (as before 3189)
• dedicated change_set_discussion table (like the mapping table before 3189)

those two changes allow us to add change_discussion (discussing individual changes), file_discussion (discussing files), *_discussion as needed.

[changeset-bot]

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Latest commit: ca9785d

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