User created observable source context

[udgover]

When users add new observables, it's correctly impossible to know who created them from the UI. The information is available from audit logs but it would help having the information as well in the UI. In order to do so, we need to add username as source in observable context like already done by feeds.

[tomchop]

Leveraging Audit logs sounds like a quick win as they are stored in a arango collection, and have a target from which we can extract an object ID. This could provide some initial context, but only for objects create from the API, and wouldn't reflect internal changes (e.g. feeds or analytics); although that could also be changed