Make activity name configurable

[magnusbaeck]

Dependencies

None.

Feature Request

The activity name (the data.name member of ActT) is currently hardcoded to be the name of the job. That might not be sufficiently unambiguous and/or descriptive for some use cases. We should provide a global configuration option that defaults to e.g. $full_job_name for backwards compatibility but that supports several other template variables so that sites can pick an activity name pattern that fits their use cases.