Allow multiple sections to connect to Objective on import.

Per [68679738](https://www.pivotaltracker.com/story/show/68679738).  Follow-up to [PR 1194](https://github.com/reciprocity/ggrc-core/pull/1194) when testing revealed that the duplicate-removing behavior was not happening for sections, due to an obsolete requirement that only one section connect to a given objective.

Sections and Clauses do not need to be treated differently since Objectives use the same connector for each.  Connection code moved to `base_row.py` to avoid having to pass section ids to importer as was done in [hotfix PR 1194](https://github.com/reciprocity/ggrc-core/pull/1183).