Several section-related fixes

* Add autocomplete field to choose Directive when creating a Section.
* Fix autocomplete search for multiple result types.
* Add `join_descriptors` for Clauses.
* Ensure `stubs()` always returns an Observe.List.
* Include Sections and Clauses into several aggregated related mappings.
* Add `Section` to LHN.
* Ensure "Map Section" and "Map Clause" show up where necessary.
* Add an `ObjectView` page for `Section` objects.
Don't allow script error when changing autocomplete option

* This was a hack to workaround recursiveness in Modals controller, but
  causes errors, so remove for now.
More Section-related fixes

* Add missing mustache template
* Re-allow creating Section in Mapping modal selector
* Don't allow creating Directive from Directive autocomplete field
Merge pull request #1153 from reciprocity/feature/fix-section-misc

Several section-related fixes