Makes Page#activity_id read-only

This should close out [#44098927] more solidly
The attr_readonly makes activity copying fail.
DRYing up the object creation methods in sg_marshal.
Cleaning up "pages change activities" bug

This is a test to verify that [#44098927] is fixed and should stay fixed, along with some notes in comments about what I tried which might be useful in the future.
Fixes a deprecation warning when testing for RAILS_ENV
WIP on class methods to count users and activities
Added authoring demo link and updated copyright year.
Better methods for getting User and Activity numbers
Merge branch 'master' of github.com:concord-consortium/smartgraphs-authoring
Better count method for User#registered_since
Quick inventory numbers for admins
Merge branch 'master' into staging