[FIX] web: fix view manager header height form view

in form view, the header height was higher than in other views (4 extra
pixels). This was caused by the floating buttons in
.oe-view-manager-buttons.  This commit forces the column to take exactly
30 px regardless of its content. (it is not a problem even if the
content takes extra lines)
[FIX] mail_tip: prevent blank webclient
[ADD] website_sale_coupon : allows to give a coupon to customer, with validity, available for some products