Feature/map log fix5

Resubmit of [PR 495](https://github.com/reciprocity/ggrc-core/pull/495) to handle the case when directives are deleted.  Event log no longer does a database hit for to get the mapped directive for Sections and Controls.

Solution is to set up `log_json` method (renamed from `to_json`) so that it passes an extra parameter that contains the parent directive (for the case those two classes) and thereby indicates that this mapping should be mentioned in the log description.