Add "gl-{number}" to reference issues and MR's

Description

Currently, GitLab uses "#" and "!" to reference issues and MR's. On GitHub, they also accept "gh-{number}" to reference issues and PR's. The rationale is that if a project is ever migrated from GitHub to another project hosting service (like this one), issue labels remain distinct because of this prefix. This allows for clearer documentation and project tracking.

If this proposal is deemed acceptable, I would be happy to try to put together an MR for this. If someone could direct me to where the Markdown is parsed, that would be great!

Proposal (updated based on discussion bellow https://gitlab.com/gitlab-org/gitlab-ce/issues/24159#note_55079243)

• You can use gli-123 to refer to a GitLab issue on the same project.
• You can use glmr-123 to refer to a GitLab merge request on the same project.
• These work in the same way as other references to those objects, but the link text is not changed to the 'standard' format, as described in https://gitlab.com/gitlab-org/gitlab-ce/issues/24159#note_54915715

Links / references

If you have a GitHub account, you can corroborate what I described above.