Restructure 2020 API to better organize assessment APIs

[eric-jahn]

Restructure 2020 API to better organize assessment APIs
The assessmentresults and assessmentquestions should be under assessments API.

The API path should be /clients/{clientid}/enrollments/{enrollmentid}/assessments/{assessmentid}/assessmentquestions
but it is
/clients/{clientid}/enrollments/{enrollmentid}/assessmentquestions

Likewise for assessmentresults APIs.