submission: create new "observer" role with password protection

[GraemeWatt]

In addition to the existing "uploader" and "reviewer" roles, it would be useful to define an "observer" role with permissions only to view a submission in preparation (and associated review comments) but not to upload or review a submission. The "observer" could be a single user, but there should also be an option to specify a CERN e-group like [email protected] as an "observer". If the latter, the code should check if an individual HEPData user (logged in with CERN authentication) is a member of the CERN e-group before giving them permissions to view an unfinished submission. The "observer" should not be included in email notifications in order to avoid spamming a large e-group (which might anyway not have permissions to receive email from outside CERN). This is a request from various ATLAS members to allow password-protected collaboration-wide access to HEPData records in preparation.