Added index for group_id_datetime on sentry_event.

When running with a highly populated Postgresql database, the query for previous event takes unacceptably long time to fail if there are no previous instances (9-10 mins for our server). The index not only solves this problem, it speeds up next/previous event queries in general.
Merge pull request #1196 from umutgultepe/patch-1

Avoid unnecessary queries on next/previous events.