Edit/View privileges of shared projects

[forrestbao]

Currently, shared projects, do not have privilege management. At least two levels are needed: Edit and View-only. For example, I share a project to Hebi and he can only view it. This can be really helpful before CodePod supports version control or history, to prevent unexpected edits to code due to unintended keystrokes.

Preferably, we should have three levels: Owner, Editor, Viewer.

For public projects, two levels: Editor and Viewer.

Just like in Google Docs.