Add support for link type between issues

[jmini]

Fixes #847

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In https://docs.gitlab.com/ee/api/issue_links.html#create-an-issue-link

POST /projects/:id/issues/:issue_iid/links

Add link_type parameter in the request and attribute in the response

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In https://docs.gitlab.com/ee/api/issue_links.html#list-issue-relations

GET /projects/:id/issues/:issue_iid/links

Add missing attributes to Issue:

    "link_type": "relates_to",
    "link_created_at": "2016-01-07T12:44:33.959Z",
    "link_updated_at": "2016-01-07T12:44:33.959Z"

There was already "issue_link_id"

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This PR prepares an AbstractIssue class, this contains the common fields between:

• Issues
• Linked issues
• Epic Issues (see EpicApi.getEpicIssues() returns models.Epic not models.EpicIssue #729)

Object tree would be:

• Issue continues to extend AbstractIssue
  • Just subscribed would stay on the concrete Issue type.
  • The attribute from the linked issue would be removed (this is a breaking change can be done with 6.x.x).
• LinkedIssue would have (also for 6.x.x):
  • issue_link_id
  • link_type
  • link_created_at
  • link_updated_at
• The class EpicIssue (see EpicApi.getEpicIssues() returns models.Epic not models.EpicIssue #729) would also extends AbstractIssue with following attributes:
  • epic_issue_id
  • relative_position

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On commit jmini@f63e2d9

I have tried a path where LinkedIssue would be a subtype of Issue to preserve backward compatibility.

But you can still not change the return value of method method getIssueLinks(..) from List<LinkedIssue> to List<LinkedIssue>.

gitlab4j-api/src/main/java/org/gitlab4j/api/IssuesApi.java

Line 836 in 2d2fe24

public List<Issue> getIssueLinks(Object projectIdOrPath, Long issueIid) throws GitLabApiException {

But even if LinkedIssue is a subtype of Issue, this is a breaking change.

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So changing the return type getIssueLinks(..) to LinkedIssue and refactoring Issue is something that could be done in 6.x.