allow pinning plugins to specific commit
allow pinning plugins to specific commit

refs DE-169

[pin-commit-analytics=87e16aa4f40bf1e2db93aa3acfd7ccde8394b3cb]

Change-Id: Ib299df736881e591e43818bdbc35fbc1948281a1
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/243973
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: James Butters <4c5a489c9bb72d41f20b3b22aaf4c9b864e32f20@instructure.com>
QA-Review: James Butters <4c5a489c9bb72d41f20b3b22aaf4c9b864e32f20@instructure.com>
Product-Review: Aaron Ogata <11ccf682c06d1508642f943a3962a1c3d00b8e44@instructure.com>

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