Improve "Available Simulations" endpoint

[jjnesbitt]

For some reason, the endpoint used to retrieve available simulations per context is taking exceptionally long (around 20 seconds). This is at the endpoint:

http://localhost:8000/api/v1/simulations/available/context/{context_id}

It seems to make 75 SQL queries, which indicates to me that something unintentional is going on. This should be investigated and addressed.