Support actors as first-class entity

[behas]

Currently, tags carry labels / strings; it should also be possible to link a tag with an actor (e.g., some VASP), which is defined as first-class entity.

[cybersec-code]

Pull request #66 solves the first step in this task (supporting ActorPacks); we still need to define how to integrate TagPacks with ActorPacks (e.g. replacing some labels with actors)

[mdragaschnig]

After discussing the integration of actors with tags:

• actors are statically defined by tagpack creators; in a dedicated optional "actor" field in the tagpack file
• the tagpack-tool offers creators of tagpacks some support for finding a suitable actor: e.g. for a given label, or labels in a given tagpack file, a new tagpack command suggest_actors_for_tags checks in the database for suitable actors, and prints the information/suggestions to the console, or writes it to a file, or maybe even creates an updated tagpack accordingly
• the tagpack creator checks the tagpack for correctness
• actor information (if present) in tagpacks is inserted into the database along with the other tag infos