[IMP] account: show send by email button on invoice for pro-forma stage too

- when we send invoice by email, the template shows `False` at invoice number,
we will show `pro-forma` invoice instead of `False`

addons/account/data/email_template_data_invoice.xml

@@ -25,7 +25,12 @@
     (<i>${object.partner_id.parent_id.name}</i>)
 % endif
 ,</p>
-<p>Here is your invoice <strong>${object.number}</strong>
+<p>Here is your 
+% if object.number:
+invoice <strong>${object.number}</strong>
+% else:
+pro-forma invoice
+% endif
 % if object.origin:
 (with reference: ${object.origin} )
 % endif

addons/account/views/account_invoice_view.xml

@@ -347,9 +347,9 @@
             <field name="arch" type="xml">
                 <form string="Invoice">
                 <header>
-                    <button name="action_invoice_sent" type="object" string="Send by Email" attrs="{'invisible':['|',('sent','=',True), ('state', '!=', 'open')]}" class="oe_highlight" groups="base.group_user"/>
+                    <button name="action_invoice_sent" type="object" string="Send by Email" attrs="{'invisible':['|',('sent','=',True), ('state', 'not in', ('open', 'proforma', 'proforma2'))]}" class="oe_highlight" groups="base.group_user"/>
                     <button name="invoice_print" string="Print" type="object" attrs="{'invisible':['|',('sent','=',True), ('state', '!=', 'open')]}" class="oe_highlight" groups="base.group_user"/>
-                    <button name="action_invoice_sent" type="object" string="Send by Email" attrs="{'invisible':['|',('sent','=',False), ('state', '!=', 'open')]}" groups="base.group_user"/>
+                    <button name="action_invoice_sent" type="object" string="Send by Email" attrs="{'invisible':['|',('sent','=',False), ('state', 'not in', ('open', 'proforma', 'proforma2'))]}" groups="base.group_user"/>
                     <button name="invoice_print" string="Print Invoice" type="object" attrs="{'invisible':['|',('sent','=',False), ('state', '!=', 'open')]}" groups="base.group_user"/>
                     <button name="%(action_account_invoice_payment)d" type="action" states="open" string="Register Payment" groups="account.group_account_invoice" class="oe_highlight"/>
                     <button name="action_invoice_open" type="object" states="draft" string="Validate" class="oe_highlight" groups="account.group_account_invoice"/>