Feature: Import objective to directive

Per story [59598318](https://www.pivotaltracker.com/story/show/59598318).  Fix of [PR 645](https://github.com/reciprocity/ggrc-core/pull/645) that was easier to do as a new branch. 

Allows import of objectives to directives with the following specification:

- If a valid slug is given in the section/clause column, imported objective connects *only* to the section/clause, not directly to the objective.
- If a valid slug is given in the control column, it is connected to the newly imported objective.

Also includes fix for problem of linked objects being rendered as "XXX" in the import table interface, story [61086596](https://www.pivotaltracker.com/story/show/61086596).  Was due to a brittle check in the `_import_cell_.haml` template for whether the base class is `LinksHandler`, which behaved incorrectly if the class was derived deeper than one level.