Extract discussions data from notes table into its own

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Currently, discussions are built in the runtime from notes of the discussion: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/models/discussion.rb#L41. Also, some columns in the notes table contain the data related to the whole discussion and this data is replicated across all the notes of the discussions. For example, line_code, position, original_position, change_position.

If we could move the discussions data into its own table, we would duplicate this data. That would help us to solve the following problem: #213854 (closed)

Related issues/comments:

• #198457 (closed)
• !28623 (comment 320941355)