Previously imports would fail if a merge request note included a
suggestion with an
`ActiveRecord::HasManyThroughCantAssociateThroughHasOneOrManyReflection`
exception.

This was happening because suggestions were listed as a descendant of
merge requests, but this doesn't work because suggestions are directly
associated with notes, not merge requests, and that association is lost.
Rails also disallows creating intializing a has_many association through
a different object.

We fix this by making `suggestions` a child of `notes` within a merge
request. This doesn't fix previously broken exported project exports,
but new exports will work.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/65880