CLC-4486 - Added canvas setting for official courses sub-account ID
CLC-4486 - Refactored Canvas::ExternalTools so that #public_list and #public_list_as_json are class methods which return LTI application IDs from both the root account and 'Official Courses' sub-account
CLC-4486 - Updated CanvasController#external_tools to use new interface to Canvas::ExternalTools.public_list_as_json
Merge pull request #3133 from redconfetti/CLC-4486-official-courses-ext-tools

CLC-4486 - eGrades Export Tool: Update External Tools API to include tools from 'Official Courses' sub-account