Minor fixes for some trees

* add missing + Object links
* specify list_view for Person, Document
* place {{#if viewLink}} and {{#if mappings}} correctly
Initialize widgets using JoinDescriptor specifications

* add GGRC.ListLoaders.ProxyListLoader
* remove Controllers.PageObject#init_related_widgets and friends
* add JoinDescriptor#get_list_loader
Don't show risk; don't use SystemControl
Change signature of Resource#get_properties_to_include

* preparing for eager-loading associations in ObjectView
Enable overriding or suppressing creation of some widgets
Fix overridden descriptors and add cases for Program
Add DirectListLoader and apply to Directives

* Directive#sections, Directive#controls
* fix more miscellaneous tree issues
* add 'unmap_or_delete' helper to check whether 'unmap' operation will
  delete the object itself
Move 'Relatable' out of 'Base'

* `type` was not being set correctly on models with single table
  inheritance (System/Process and Directive types)
Deprecate 'SystemControl' and 'SystemSystem' tables

* in favor of using ObjectControl and Relationship tables
Change multi-type selector to load incrementally

* removes live binding-to-list on "option" column in favor of appending
  elements manually (to avoid full-list redraw on increment)
* add basic support for search (no interface support yet)
Enable semi-lazy loading in LHN lists
Change ListLoaders and show more links in more places

* ListLoaders are now model-oriented, with lightweight bindings to instances
Add 'Objectiveable' to Objectives, and rename association

* this enables Objectives to map to other Objectives
Debounce internav updates to not delay AJAX requests
Merge remote-tracking branch 'origin/develop' into feature/more-mappings
Merge branch 'develop' into feature/more-mappings

Conflicts:
	src/ggrc/assets/javascripts/mustache_helper.js
Merge pull request #266 from reciprocity/feature/more-mappings

A bunch more mappings and removal of special case mappings