Add missing index for getting recent push events

What does this MR do?

This adds an index I forgot to add in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13995 and that is necessary to ensure optimal performance.

This MR should be picked into 10.0 so customers aren't hit by this problem. The index already exists on GitLab.com (I added it manually), hence the index existence checks are in place.

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together