Fix - message sent to a list not associated with a campaign couldn't be shown in archive - to know which message to show even if the list is not at the campaign, we store test messages in table test_messages
WiP on selectable mosaico templates.
TODO: Make files always point to trusted URL, such that we don't have to rebase them. They are public anyway. The same goes for mosaico endpoints: /mosaico/templates and /mosaico/img