Use task subclass to new up new task

Fixes this issue of staging

```
Nov 12 14:06:41 tahi-staging app/web.1:    Parameters: {"task"=>{"decision_letters"=>nil, "paper_decision"=>nil, "paper_decision_letter"=>nil, "completed"=>false, "paper_title"=>nil, "role"=>"editor", "title"=>"Register Decision", "type"=>"StandardTasks::RegisterDecisionTask", "qualified_type"=>nil, "phase_id"=>"84", "paper_id"=>"20", "lite_paper_id"=>nil, "card_thumbnail_id"=>nil}} 
Nov 12 14:06:42 tahi-staging app/web.1:  ActiveRecord::UnknownAttributeError (unknown attribute: paper_decision): 
Nov 12 14:07:00 tahi-staging app/web.1:    Parameters: {"task"=>{"decision_letters"=>nil, "paper_decision"=>nil, "paper_decision_letter"=>nil, "completed"=>false, "paper_title"=>nil, "role"=>"editor", "title"=>"Register Decision", "type"=>"StandardTasks::RegisterDecisionTask", "qualified_type"=>nil, "phase_id"=>"84", "paper_id"=>"20", "lite_paper_id"=>nil, "card_thumbnail_id"=>nil}} 
Nov 12 14:07:01 tahi-staging app/web.1:  ActiveRecord::UnknownAttributeError (unknown attribute: paper_decision): 
Nov 12 14:07:11 tahi-staging app/web.1:    Parameters: {"task"=>{"decision_letters"=>nil, "paper_decision"=>nil, "paper_decision_letter"=>nil, "completed"=>false, "paper_title"=>nil, "role"=>"editor", "title"=>"Register Decision", "type"=>"StandardTasks::RegisterDecisionTask", "qualified_type"=>nil, "phase_id"=>"84", "paper_id"=>"20", "lite_paper_id"=>nil, "card_thumbnail_id"=>nil}} 
Nov 12 14:07:11 tahi-staging app/web.1:  ActiveRecord::UnknownAttributeError (unknown attribute: paper_decision): 
Nov 12 14:07:50 tahi-staging app/web.1:  ActiveRecord::UnknownAttributeError (unknown attribute: paper_decision): 
Nov 12 14:07:50 tahi-staging app/web.1:    Parameters: {"task"=>{"decision_letters"=>nil, "paper_decision"=>nil, "paper_decision_letter"=>nil, "completed"=>false, "paper_title"=>nil, "role"=>"editor", "title"=>"Register Decision", "type"=>"StandardTasks::RegisterDecisionTask", "qualified_type"=>nil, "phase_id"=>"84", "paper_id"=>"20", "lite_paper_id"=>nil, "card_thumbnail_id"=>nil}} 
```