With the introduction of supplier invoice templates in Dolibarr 16, with the
same hook but different table, this module tried to load the customer invoice
having the same id than the supplier invoice template being treated. This could
result in severe information disclosure.
Fixes gh-10
Technical reminder about the DB upgrade :
There doesn't seem to be an easy and reusable way to give CMail both
parts of an text+html email, so we rework the database schema to have
a simple 'body' field with a 'body_ishtml' switch, following CMail
interface.
Enhancement from issue #1
