Add remote issue links to JIRA

It would be nice to improve our JIRA integration. In addition to the existing integration we could add remote issue links in JIRA based on mentions and merge requests. The JIRA API allows this easily and it can probably be hooked in to our existing cross-reference and issue transition logic.

Screen_Shot_2016-02-05_at_3.41.36_PM

Here is a list of the available fields we can send: https://developer.atlassian.com/jiradev/jira-platform/guides/other/guide-jira-remote-issue-links/fields-in-remote-issue-links

Here is the API guide: https://docs.atlassian.com/jira/REST/latest/#api/2/issue-createOrUpdateRemoteIssueLink

cc/ @JobV What do you think?