[IMP]Website Blog: Make Onboarding tour user friendly and change customi...

...ze cover feature

- Task: https://www.odoo.com/web?debug=#id=8996&view_type=form&model=project.task&menu_id=3942&action=327
- Pad: https://pad.odoo.com/p/r.MnhQk73bZbMtu2Mv
@rim-odoo  
In "cover_properties" field storing the values of background-image,color ,opacity of cover and class value to decide cover size in json format.(This all about cover formating) 
And "cover_properties_json" is function field to get the jsonified data and then pass them in template to render on page.