Add disclaimer and require acknowledgement before trading

[mross-ua]

We need add a disclaimer of liability and ask the user to acknowledge their responsibility in interacting with the platform through the API. Note that not only can there be an issue client-side but also server-side. (You'll see there are questions and bugs in the official Python client project.)

You can just look at the GPL which includes such a notice.

This must be done before any trading activity is allowed but even just monitoring positions could cause someone to act, so it might be best to show on first login. A preference should be added to remember this acknowledgement and stored in the preferences file.

Tasks

• Remember acknowledgement in user config

Dependencies

• Save user preferences to file #37
• Add portfolio API endpoints #61
• Use a designer for building UI #12