create method to find active tool_proxies for a context

fixes PLAT-640

test-plan:
specs should pass

Change-Id: Ia958982b1ff786dbafbaa88e3c080a4b61627d04
Reviewed-on: https://gerrit.instructure.com/40969
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: Nathan Mills <nathanm@instructure.com>
Product-Review: Nathan Mills <nathanm@instructure.com>