db_schema: system_info.results.overall -> results

[lyuyangh]

part of #433
fixes #320

This is the last PR for the first step. It moves overall metrics out of system_info and stores something like this for each system.

"results": {
  "example": { "rouge1": 0.33, ...},
  "token": { "F1": 0.35, ... }
}

Previously, the endpoints and the UI do not specify the analysis level for each metric which can be confusing because metrics can have the same name for different analysis levels. This PR fixes this issue partially.

• Now, the systems table page displays the levels. The corresponding endpoint (GET /systems) is also updated to include this information.

The headers become even more verbose. We can group the column headers to make it easier to read but I'll leave it for another PR.

• The system submission UI and the endpoint (POST /systems) remain the same because updating those would break the python client. Also, the SDK does not support the configuration of other analysis levels at the moment.

[neubig]

Looks great! Just noticed a potential version number clash.