Summary:
New environmental variable GOOGLE_DRIVE_SCOPE that modifies the scope
requested for Google Drive integration.
For prod it has value https://www.googleapis.com/auth/drive.file which leaves
current behavior (Grist is allowed only to access public files and for private
files - it fallbacks to Picker).
For staging it has value https://www.googleapis.com/auth/drive.readonly which
allows Grist to access all private files, and fallbacks to Picker only when the file is
neither public nor private).
Default value is https://www.googleapis.com/auth/drive.file
Test Plan: manual and existing tests
Reviewers: dsagal
Reviewed By: dsagal
Subscribers: dsagal
Differential Revision: https://phab.getgrist.com/D3038
