Add new index to users over deleted_at and location_id

[uberbrady]

Some location-based queries that join to Users, when there are lots of users, can be slower than they need to be, so this adds an index.

One thing we're doing that's a little weirder than usual is that we're indexing deleted_at first, then doing location_id. This is because we could probably use an index over users deleted_at at some point, and this index will cover that. But we're never going to ask for "all users from this location regardless of whether they're deleted or not" - we always ask for either deleted_at IS NULL or deleted_at IS NOT NULL.