Add commit message to JIRA mention comment

Zendesk issue: https://gitlab.zendesk.com/agent/tickets/13677

If a JIRA issue is mentioned in a commit, add the commit message to the JIRA comment from GitLab. This could look something like:

USER mentioned this issue in abcd123:
“Git commit message”

Moving here from gitlab-org/gitlab-ee#50 and gitlab-org/gitlab-ee#124

Milestone changed to 8.6

Added promised label

mentioned in issue #12490 (moved)

Added ~26362 label

Added ~122770 label

Reassigned to @rdavila

mentioned in merge request !2992 (merged)

Status changed to closed by merge request !2992 (merged)

mentioned in merge request !3021 (merged)

mentioned in commit 6f85eb38

Status changed to reopened

mentioned in merge request !3067 (closed)

Status changed to closed by merge request !3021 (merged)

We use the JIRA integration heavily and this will be greatly appreciated.

@AndyNic it will be shipped soon in the next 8.6 version