Improve "Issue was closed by" notifications when issue closed using "Fixes" commit message

Seen in the issue:

Robert Speicher @rspeicher closed via merge request !16182 (merged) (merged) 3 minutes ago

Seen in the email notification:

Issue was closed by Robert Speicher

Proposal

Improve the email notification by:

1. Including the cause for closing the issue.
2. Making things clickable (author, issue, MR/commit).

MR:

Robert Speicher @rspeicher closed gitlab-org/gitlab-ce#28916 via merge request gitlab-org/gitlab-ce!16182 (merged)

Commit without MR:

Damian Nowak @nowaker closed this/that#123 via commit d5c12a9.