Purpose

• Improve Gitlab interface implementation to handle previously defined release & update it
• Improve overall unit testing for HVCS

Rationale

Brings GitLab more in line with the features of GitHub to be able to edit a previous release's notes via thechangelog subcommand.

I also found that with the switch to usingexample.com as the domain there were more errors in the test suite as I would receive a 404 that would cause testing to fail. Clearly this means we were actually touching the real gitlab so I mocked out those appropriately.

How I tested

I tested manually with a project access token on my local machine with a gitlab project. I first ran the pipeline to create a release normally and then usedsemantic-release changelog --post-to-release-tag v1.0.0 to update the release notes with a new template.

Closes:#727