add gist index to pseudonyms to improve user search

refs CNVS-15212

test plan
 - rake db:migrate should succeed
 - user search should work and be faster

Change-Id: I8af5a1928e315a8ec68ea7b2b580a28d961c4147
Reviewed-on: https://gerrit.instructure.com/40690
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>