feat(db): add index on memberships(user_id)

One of the queries having taken the longest total time on our production
DB is finding the organizations for a given user. This index should
speed that query up significantly.
fix(db): add newlines to membership-indices migration

My editor got borked and inserted carriage returns instead of newlines.
fix(db): update structure.sql file

I was missing a migration locally, so the migration was removed from
structure.sql in 80b5e0c720ef9b6e25ba9253892ee77216285c3f. This commit
adds that migration back.
Merge pull request #349 from travis-ci/hh-index-memberships

feat(db): add index on memberships(user_id)