Add syllabus body field to course graphql schema

[lukekrikorian]

Adds the additional syllabus_body field to the graphql endpoint for courses.

I'm working on an external frontend client for canvas, and hoping to switch to graphql for some of the backend calls, but there are some fields missing from the REST alternative. Apologies if this PR is malformed, I'm not very familiar with this stack.

Test Plan:

• Create a new course

• Add a syllabus to the course

• Navigate to https://<canvas_host>/graphiql and enter the following query:

  query MyQuery {
      course(id:COURSE_ID_HERE) {
          syllabusBody
      }
  }
  

• Ensure that the result returns the syllabus body HTML.

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