Add time indexes to Submission model to improve performance

[TimJentzsch]

Many Buttercup queries filter submissions by ranges of create_time, claim_time or complete_time.
In particular, the /queue command fetches submissions filtered by ranges of these dates, every 2 minutes.

We should add tree indexes for these fields to make these queries faster.
I suggest a clustered index for create_time and unclustered indexes for claim_time and complete_time.