Add url, approved_by, and merged_by to merge template

Merged Lucas Zampieri requested to merge lzampier/gitlab:new_fields_on_merge_template into master

What does this MR do?

Adds more fields to merge template:
url, full mr URL, the main difference to reference, it that reference can only be accessed from within GitLab.
approved_by, a line-separated list of the users who have approved the mr.
merged_by, name and email of the user who merged the mr.

Related issues

Fixes: POV-99

Edited by Lucas Zampieri