Allow categorizing jobs into different categories/ buckets for runners to pull from

[helegraf]

Currently, any runner can pull any open experiment from the database. However, not all experiments may have the same resource requirements (e.g. requiring/ not requiring GPU, different runtime, etc. which on a cluster would be executed by scripts with different limits). It would be practical to assign jobs a category, and set runners up so they can only pull jobs from that category as an optional feature (default category to keep current functionality). Maybe this could be realized by a kind of tagging feature?

[tornede]

This is closely related to changed required by issue #19