Clean up ProjectsController

Add JSON-Schemas for create and update methods

Started out working on #320 before I realized it was asking for something different. Ended up cleaning up somethings in the controller anyway. 
is there any way to share the content of the create/update schema? Seems that most of the code is duplicated apart from 
```
property "owner" do
  type "object"
  required "id", "type"

  property "id" do
    type "string"
  end

  property "type" do
    type "string"
  end
end
```