Seed abuse reports

```
rake db:seed_fu FILTER=abuse_reports
```

Thanks to @stanhu,
https://gitlab.com/gitlab-org/gitlab-ce/issues/28059#note_23325328

changelogs/unreleased/seed-abuse-reports.yml

+---
+title: Seed abuse reports for development
+merge_request:
+author:

db/fixtures/development/18_abuse_reports.rb

+require 'factory_girl_rails'
+
+(AbuseReport.default_per_page + 3).times do
+  FactoryGirl.create(:abuse_report)
+end