fix csv exporting when user can view but not edit grades
fix csv exporting when user can view but not edit grades

fixes CNVS-21040

Users who have view permissions but do not have edit permissions should
be able to export the gradebook to a CSV file. (For example, TAs might
have these permissions)

Previously, there was an "Export" button, but it would cause an error
when clicked

Test plan:
* Create or edit a role in which users can view grades but cannot edit
  grades
* Enroll a user with this role in a course
* Ensure that this user is able to export the gradebook to a CSV file

Change-Id: I2494bb8598ffb70bf55303c0b589f03213ec6d99
Reviewed-on: https://gerrit.instructure.com/56980
Tested-by: Jenkins
Reviewed-by: Dylan Ross <7374864f039b76a5b01d8407f92828e3f173309e@instructure.com>
QA-Review: Jason Carter <976bfc4bed0555ff96b3266aea18e7b17a9d471c@instructure.com>
Product-Review: Christi Wruck

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