[FIX] account: tax base on invoice report

Commit e1deead1cb introduces back the table of taxes. But it is still
incomplete:
- it should appear all the time
- the tax base should be included as well

Sources:
http://ccff02.minfin.fgov.be/KMWeb/document.do?method=view&id=21786bb3-69a0-4323-8631-aa898b988097#findHighlighted
http://www.ipcf.be/Uploads/Documents/doc_1704.pdf
http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2002:015:0024:0028:FR:PDF

opw-724519

addons/account/views/report_invoice.xml

@@ -118,18 +118,23 @@
                 </div>
             </div>
 
-            <div class="row" t-if="len(o.tax_line_ids) > 1">
+            <!-- DO NOT REMOVE THIS TABLE. MANDATORY IN SOME COUNTRIES -->
+            <div class="row" t-if="len(o.tax_line_ids) > 0">
                 <div class="col-xs-6">
                     <table class="table table-condensed">
                         <thead>
                             <tr>
                                 <th>Tax</th>
+                                <th class="text-right">Base</th>
                                 <th class="text-right">Amount</th>
                             </tr>
                         </thead>
                         <tbody>
                             <tr t-foreach="o.tax_line_ids" t-as="t">
                                 <td><span t-field="t.tax_id.description"/></td>
+                                <td class="text-right">
+                                    <span t-field="t.base" t-options='{"widget": "monetary", "display_currency": o.currency_id}'/>
+                                </td>
                                 <td class="text-right">
                                     <span t-field="t.amount" t-options='{"widget": "monetary", "display_currency": o.currency_id}'/>
                                 </td>