Follow-up from "Filter abuse reports by user"

The following discussion from !19365 (merged) should be addressed:

• @grzesiek started a discussion: (+1 comment)

  There are no indices on user_id and reporter_id fields in db/schema.rb. Is that going to work on gitlab.com, where the table is probably quite large?

TODO

Add indices on user_id and reporter_id to abuse_reports table