Design data structure structure to store "TODO" items for review

[pombredanne]

We need to be able to store TODO/review items (may be the same way we can store compliance alerts?)

The toolkit side has this for a start https://github.com/nexB/scancode-toolkit/issues/3122

This is to support:

• #102
• #30

[tdruez]

@pombredanne In order to design an implementation, we would need some concrete examples and variations of data for which TODO items would be created.

Also:

• When a TODO item is created? During existing pipes? New pipes/pipeline specialized to generate TODOs?
• Schema for the TODO model? Probably a mix between fields to define the issues and context, along with some fields for the resolution process
• Are there fields from https://github.com/nexB/scancode-toolkit/issues/3122 proven to be useful that we could reuse in this implementation?