Split .gitlab-ci.yml into templates

What does this Merge Request do?

Splits .gitlab-ci.yml into templates to make it a bit easier to navigate.

Behaviour of all jobs should be unchanged (however their names are no longer prefixed with the name of the stage).

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• [-] Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.