Support Lead TA course role

[alxstx]

Changes
Added Lead TA as a supported staff/course-admin role across course authorization and dashboards
Added Canvas custom role mapping so internal leadta sync uses enrollment_role=Lead TA while built-in roles keep enrollment_type[]
Updated course import filtering to recognize Canvas Lead TA enrollments as staff courses
Updated course and enrollment tables to display Lead TA with a readable label
Added regression tests covering Lead TA role support, import behavior, enrollment sync, and Canvas query parameters

Testing
Run bundle exec rspec spec/models/user_to_course_spec.rb spec/models/course_spec.rb spec/controllers/courses_controller_spec.rb spec/facades/canvas_facade_spec.rb for Lead TA role tests
Run the full rspec test suite to verify no regressions
Run bundle exec rubocop app/models/user_to_course.rb app/models/course.rb app/controllers/courses_controller.rb app/facades/canvas_facade.rb spec/models/user_to_course_spec.rb spec/models/course_spec.rb spec/controllers/courses_controller_spec.rb spec/facades/canvas_facade_spec.rb to verify changed Ruby files

Documentation
No documentation needed

Checklist

• Name of branch corresponds to story
• 80%+ test coverage with all tests passing

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[alxstx]

Closing because this PR should be opened against cs169/flextensions first.

[alxstx]

Reopening — corresponding upstream PR is now open at cs169#350.