Issues on user permissions

[stevezhang1999]

Is your feature request related to a problem? Please describe.
Currently there is only one owner/user of the project.
For normal users, it is impossible to access projects/documents from other users. Need adjustments to better work for collaboration.

Describe the solution you'd like
Currently there is only one user group (normal user, besides staff members), users from normal user could only operate their own projects.
A better design is to assign different user role (e.g. editor, viewer) to the user for each project, to make the permission system more flexible and better for collaboration. Besides, the project may have the property 'public' or 'private'.