report-image-diff: Rewrite the report image diff to use buildhistory

The old way used the gitlab UI to load the html overview for the
artefacts, but this didn't worked for private repos, as the login
was not implemented. Loading file from the buildhistory with known
names uses the gitlab API, where the credential handling is provided.

Also replace loops and duplicated code with internal classes, which
massively restructures the code.