Make GitLab Bot's label messages friendlier, more descriptive, and more consistent.

This makes GitLab Bot's label messages friendlier, more descriptive, and more consistent.

Currently, the messages are not qualified, which seems a little harsh when one does not know what labels are appropriate for an issue or a merge request.

Note: The Merge request guidelines state the following, indicating that some users may not be allowed to set labels whether or not they know which labels would be appropriate.

If you’re allowed to (Core team members, for example), set a relevant milestone and labels.