Abstract out assignments into tasks

[cormander]

As a campaign manager, I need to create tasks for volunteers so that we can provide clear instruction as to what needs to be done within the tool.

This feature is done when:

• A new task creation API exists
• Tasks require a form, a list ("turf" or otherwise), and a text description
• A task status API exists
• The "/hello" endpoint is updated to pull assignments from tasks if they exist