Add setting for activated modules and make the modules distinction even clearer

[LefterisJP]

Problem Definition

At the moment everything rotki checks, runs as the app runs. This may eventually become really expensive and time consuming due to the amount of blockchain and external service queries for each module.

Additionally we may at some point want to have premium only modules which can only be activated if the user has a premium subscription so separating modules in a clean way already is a good idea.

Task

Backend

• Add a new setting. active_modules. It's a list of strings. Each string is a module name. Valid names for now are: makedao_dsr, makerdao_vaults.
• Split makerdao.py into makerdao_dsr and makerdao_vaults and activate them only if the user has the module activated.
• Add new error code in API calls if the module is not activated.

Frontend

• Allow the user to set the active modules.
• For makerdao DSR and for makerdao vaults process the module not activated error and show a useful message to the user.

[LefterisJP]

So @kelsos this requires frontend work. The backend work has already been done.