Only create unmergeable todos onceonly-create-unmergeable-todo-once

Previously, we created an unmergeable todo when a merge request:

1. Had merge when pipeline succeeds set.
2. Became unmergeable.

However, when merge when pipeline succeeds fails due to unmergeability,
the flag isn't actually removed. And a merge request can become
unmergeable multiple times, as every time the target branch is updated
we need to re-check the mergeable status. This means that if the todo
was marked done, and the MR was checked again, a new todo would be
created for the same event.

Instead of checking this, we should create the todo from the service
responsible for merging when the pipeline succeeds. That way the todo is
guaranteed to only be created when we care about it.

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