Merge branch 'iurisilvio/gitlab-ce-bitbucket_closed' into 'master'

Fix bug where Bitbucket 'closed' issues were imported as 'opened'

_Originally opened at !2930 by @iurisilvio._

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Bitbucket has an undocumented status `closed`.

https://confluence.atlassian.com/bitbucket/issues-resource-296095191.html

Related to https://gitlab.com/gitlab-com/support-forum/issues/559

See merge request !2973