Orthograph

Orthographic correction in edit_users.html for french template.
Bug correction in Invoices class

There was an error in Invoices class, leading to an undefined result if
the user was not in the invoice.
Simplify the global payback matrix

Added a new feature. The "global payback matrix" is now simplified in
order to minimize the number of exchanges between members.
Mask invoices when user is not concerned
(except if admin user)

Only the invoices that concern the current user (as buyer or user in) are displayed now.
Added a balance statement on the index page