RFR: Issue tracker support

[Robin Westberg]

Hi all,

Please review this change that introduces issue support. An Issue contains a subset of PullRequest functionality, similar to how this is modeled in GitHub and GitLab. It also allows creating support for other issue trackers that do not have repository support, such as Jira.

Best regards,
Robin

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Commits:
 - 7b0e6107: Implement test version of Issue and IssueProject
 - b603a667: GitHub and GitLab does not yet implement Issues
 - d1886dcc: Extract IssueProject and Issue from HostedRepository and PullRequest

Changes: https://git.openjdk.java.net/skara/pull/162/files
 Webrev: https://webrevs.openjdk.java.net/skara/162/webrev.00
  Stats: 854 lines in 20 files changed: 608 ins; 240 del; 6 mod
  Patch: https://git.openjdk.java.net/skara/pull/162.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/162/head:pull/162

PR: https://git.openjdk.java.net/skara/pull/162