Clean up schema for table trending_projects

Per https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):

Missing constraint: UNIQUE (project_id)

Things to check (unclear to me):

How do we test migrations? https://gitlab.com/gitlab-org/gitlab-ce/blob/master/spec/migrations/README.md
Do we need to assume we already have duplicates and remove them ahead of time (I think we should)?

Edited Jan 31, 2018 by Andreas Brandl