Related observables count

[udgover]

This PR adds the ability to sort entities by related_obsevables_count by:

• implementing a related_observables_count method as a computed_field in Entity class. This method calls neighbors with observable target type and 1 hop distance in any direction.
• setting related_observables_count variable in AQL query in ArangoDatabase filter method. This variable executes the same query as related_observables_count Entity method and add the variable to sorting if it's defined.

Test is implemented in tests/schema/graph