heptapod-ci: turn off pipelines for merge request events These are always duplicated by the external (from the user) push, or internal (from Heptapod itself) push pipeline, so they're entirely redundant.

heptapod-ci: always make the default run condition explicit This has no effect on behavior, it's just to make everything obvious.

heptapod-ci: remove push exception for named branches The internal workings of Heptapod cause even fast-forward merges to look like pushes to Gitlab. The only way to run a pipeline on topic-less named branches is not through the web interface. I am probably the only person affected by this, so it's not really a problem. This was confirmed with Georges, the maintainer of Heptapod, though the only way to be *actually* sure, is to merge this and see.

heptapod-ci: (hopefully) fix the pipeline savings system There is no easy way for me to test this outside of the actual repo since it would require setting up `heptapod-runner` locally, making a Git repo with the right changes, etc. It's not worth my time at this point. If this fails, I'll try to ask for help from Georges, the maintainer of Heptapod.