fix substitutions_helper with observer roles
fix substitutions_helper with observer roles

test plan:
* have a teacher who is also an observer for a course
* should be able to add external tools as modules

closes #CNVS-24785

Change-Id: I76f795593597fb50cb86b685d3bb04c50cc2c737
Reviewed-on: https://gerrit.instructure.com/66682
Tested-by: Jenkins
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
QA-Review: Ryan Allen <b65b638b3333142073650bd017e5f36609bed2c6@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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