Add GraphQL query to manage Model

[syncpark]

Currently only 1 GraphQL query is implemented

• models: query to get id, name, datasource of model

To manage model from Web UI, the following query should be implemented.

• upload(or register) new model: Read model from the specified path and register it to database
• remove model: Remove model from the database
• update model: Change the datasource of model.
• Change model storage.
  And currently all models except trained model are stored in Postgres database.
  But the model over 2GB can not be stored in the column of Postgres table.
  I think it's better to manage all the models as file.

[syncpark]

Related issues:
reconverge: model size issue