Add flag/attribute/trait system

[mikron-ia]

Currently, the only ways to add a configuration variable per object is to either introduce a 0/1 flag into the database (which would clutter the record) or to use a Parameter (which is not the intended use, is hardly intuitive, and clutters a different place).

Consider adding some flag/attribute/trait system that would allow an on/off configuration per object. First use could be in #58 for display as tab for Group functionality.

[mikron-ia]

A possible solution: a 1:1 to Epic and any other object that needs it ConfigPack object containing a JSON packed-configuration, unpacked in entity, accessed via getters, with all variables expected there (fed with defaults if absent).

Alternatively, not even a separate DB entry but just a JSON field within the object (with the same name across all objects), unpacked on use and packed on save by a trait-driven mechanism with a method ensured by HasConfig interface.

Possible issues:

• Not all objects need the same flags (Display as tab for Group is a good example, nothing else needs that, perhaps except Thread once they exist) - this can be solved by getters returning false if absent regardless of sense, returning null if absurd, or just having two versions that have either behavior
• Once this solution gets values, it becomes essentially a new version of Parameter, creating a pointless redundancy - keeping it as on/off is necessary, and simplifies interface, too