Branches can have their own version of notices - added branchcode to
letter table.
Support html notices - added is_html to letter table.
Support for borrower attributes in templates.
GetPreparedletter() is the interface for compiling letters (notices).
Sysprefs for notice and slips stylesheets
Added TRANSFERSLIP to the letters

Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>