Fixes #992

Co-authored-by: Jan Henrik Wiesner <jan.wiesner@gebit.de>
Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

Changes MergeRequestApi:

- added methods "getDiffs()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/diffs

- add methods "getDiffVersions()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/versions

- add method "getDiffVersion()" and "getOptionalDiffVersion()" for GitLab Endpoint:
GET /projects/:id/merge_requests/:merge_request_iid/versions/:version_id

- deprecate "getMergeRequestDiffs()" and "getMergeRequestDiff()"

Fixes #993

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Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

* Add methods to create child epics

* Add methods to manage related epics

* Fix Epic Issue link association

Fixes #729

* Change return type

* Added ReleaseLinks API (#639)

* Deprecate the "external" attribute in Link

* Add tests

* Rename to ReleaseLinksApi and add the getter in GitLabApi

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Co-authored-by: Jeremie Bresson <jeremie.bresson@unblu.com>

* Add ExternalStatusCheckApi

Fixes #919

* Add ExternalStatusCheckEvent

* Fix javadoc

Remove UserListSerializer and UserListDeserializer from approvedBy field in ApprovalRule (fixes #589) (#590)