ACL: check user has access to current workspace

[ Feature description ]

The user must whether:

• has a direct permission accessLevel for this repository ;
• be a member of a group with such a permission.

The permission flag must be greater than 0.

IF this isn't satisfied, the access is denied.

[ Core libraries changes ]

• New method: Workspace::userCanAccess (User $user) Determines if the specified user has access to the current workspace.

• User:get_workspaces() keeps the result cached in class private User::$workspaces property.

[ Notes ]

• A coming commit will convert underscore method names to camelCase ones.

• The permission flag is intended to be 0, 1 or 2:
  • 0: access revoked or suspended
  • 1: regular access, with the possibility to specific rights by other permissions
  • 2: workspace system administrator access
    
    The only technical enforcement done is > 0.