make resolve_path API operate relative to course root

fixes CNVS-13260

test plan:
 - in course, group, and user context:
  - ensure /api/v1/{context}/folders/by_path alone returns
    an array containing only the context's root directory
    (usually named "course files" or "my files", etc.)
  - ensure /api/v1/{context}/folders/by_path/{path} returns
    an array containing the context's root directory
    followed by the folders in the path

Change-Id: Iecf6de0aed301041f8aa55b3d13123e0c503844e
Reviewed-on: https://gerrit.instructure.com/35352
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>