Set agrees_to_tos example text to link to site-wide ToS (or a warning message).
Also, since a renaming import statement is being used already, change
user_logic.logic to just user_logic.
Patch by: Todd Larsen
Review by: to-be-reviewed
Do not crash on *every* page load if there is no ToS Document.
'tos' ReferenceProperty might be present, but still be None.
Patch by: Todd Larsen
Review by: to-be-reviewed