save_changes_to_origin should specify origin

[gfodor]

the meta tag save_changes_to_origin is problematic because we want to enable forking but we don't want this flag to propagate to forks, since it will be a catch-22: you won't get the save dialog if you fork it when this flag is on, but we don't want to show 'save changes' on many sites.

to fix it, the flag should be disable_save_changes_for_origin and allow multiple, and it should have a content value of the origin. if that tag is set, then we consider saving changs to be false, and we write the origin of the current site we are on.

when the page loads, we clear out all of the meta tags that don't match a substring of the current origin.