Use internal templates in getChangeMessageThreadId, and to set
the listId headers. These are not admin editable templates
(they are not in files), they simply make the code a bit more
readable and editable at very little cost since we already
support velocity templating in emails. Additionaly, factor
out the fromline creation to its own function.
Change-Id: I197b239e657e1dd604b07510c4430675f05d84eb
6d28ded562