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Discussion.override_discussion_id

When notes on a commit are displayed in context of a merge request that contains that commit,
these notes are to be displayed as if they were part of one discussion, even though they were actually
individual notes on the commit with different discussion IDs, so that it's clear that these are not
notes on the merge request itself.

To turn a list of notes into a list of discussions, they are grouped by discussion ID, so to
get these out-of-context notes to end up in the same discussion, we need to get them to return the same
`discussion_id` when this grouping happens. To enable this, `Note#discussion_id` calls out
to the `override_discussion_id` method on the appropriate `Discussion` subclass, as determined by
the `discussion_class` method on `Note` or a subclass of `Note`.

If no override is necessary, return `nil`.
For the case described above, see `OutOfContextDiscussion.override_discussion_id`.

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