As a Project Admin I want to identify content available for the entire Project (for use across all deliverables)

see drag and drop files to add sources
https://projects.invisionapp.com/d/main/default/#/console/12114617/255156170/preview

Acceptance criteria:

• I can upload a binary using the drag and drop
• The uploaded binaries are Asset documents
• Assets are located in the Source container of the Project

Out of scope: Creation form of an Asset