Issues on user permissions

**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'.

No files, tests, or entry points are named. Start by mapping the existing user and project permission model, then define the collaboration design: per-project editor and viewer roles plus public or private project visibility; done requires an agreed permission model and corresponding behavior.

Written by the indexing model from the issue text.