determine if a user is a teacher for dashcards based on visible enrollments
determine if a user is a teacher for dashcards based on visible enrollments

not based on if they have a teacher role _in the current account_

fixes FOO-2224

test plan:
 * enroll as teacher in account A
 * go to the dashboard in account B (where you have no enrollments)
 * it should show Published and Unpublished sections

Change-Id: I7d2670a4eb0c621a3178024259a3632195f3444f
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/270660
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: Nate Armstrong <619b324efbf5f7848de055bdc8bef4c0dd7c8ef5@instructure.com>
QA-Review: Nate Armstrong <619b324efbf5f7848de055bdc8bef4c0dd7c8ef5@instructure.com>
Product-Review: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>

sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms