Flow query

I leave a baby on your doorstep.

- The scopes for retrieving a users flows are now in a `FlowQuery` class.
- `FlowSerializer` and its ilk just use the lite papers and tasks from the `FlowQuery`.
It looks like invoking `flow_map` causes all flows to be calculated, since in order to return the map, all values are determined. I wonder whether something like this can work:

```ruby
def tasks
  @tasks ||= flow_map[flow_title].call
end

def flow_map
  {
    'Up for grabs' => -> { unassigned_tasks }
    'My papers' => -> { paper_admin_tasks_for_user },
    'My tasks' => -> { assigned_tasks.incomplete },
    'Done' => -> { assigned_tasks.completed }
  }
end
```