Store submissions as json documents

[spolnik]

It make sense to use schema-less format to store submissions. Firstly, submissions should
be stored with full info necessary to judge it and to generate ranking from it. The question is:

• how much of user and how much of problem do we need to know?
• How much can be easily recalculated vs need to be retrieved?

Some useful info on how to store json within Cockroach DB:

• https://www.cockroachlabs.com/docs/stable/demo-json-support.html