Investigate student-tasks API call for potential optimization

[themightychris]

Spikes in student-tasks API calls (volume and time consumption) seem to be associated with periods where Slate's availability becomes degraded. It is unclear if this is the cause or just a symptom. NewRelic data seems to indicate a lot of load on the submissions table, so it may be the case that we have an inefficient subquery or PHP enrichment loop that scales poorly with volume of student tasks data.

We should analyze the characteristics of loading the teacher tasks dashboard against a highly populated database and look for opportunities to reduce query volume and optimize any slow queries