Directly link objects with sources

[julianbrost]

Currently, objects have no direct relations with a source but are only indirectly linked to sources via events. That means that multiple sources can submit events for the same object at the moment, where the idea was to allow the same incident to be fed with events from multiple sources. However, that's quite a strong assumption and we'll want incidents covering multiple objects anyways, so it makes sense just doing the same for the "same" object from multiple sources (these will be considered different objects after this change, that's why same is in quotes).

Tasks

Every step has an implicit "adapt the Go code accordingly" attached to it:

• Move the source_id foreign key from the event table to the object table (TBD: include in primary key or in object.id hash?).
• This makes the source_object table obsolete, move the information from it to the object table.
• After review for schema and Go changes: coordinate corresponding changes in PHP.