allows SIS Agent Token to show on Allowed feature option

fixes SIS-699

Test plan:
- go to settings
- click the 'Feature Options' tab
- go to the 'Post to SIS' feature and click 'Allow'
- go back to the settings tab
- refresh
- confirm the the SIS Agent Token field is present
- go back to te feature option and turn it off
- go back to the settings page
- refresh
- confirm the option is no longer there

Change-Id: I76a488938a238f116da162192a4a12c1a926e01c
Reviewed-on: https://gerrit.instructure.com/45191
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Tyler Pickett <tpickett+gerrit@instructure.com>
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>