fix duplicate notification policies

 * use a join, not an include, when looking for channels by frequency
 * make the index more specific and unique

Change-Id: Ie8b83144e5155241ff22e40519584ffe5314541b
Reviewed-on: https://gerrit.instructure.com/39442
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>