Add FinancialLinks

For now this is only usefull for plugins, since there is no UI.

app/views/finance/invoices/show.html.haml

@@ -65,6 +65,10 @@
 %p
   %b= heading_helper(Invoice, :note) + ':'
   =h @invoice.note
+- if @invoice.financial_link
+  %p
+    %b= heading_helper(Invoice, :financial_link) + ':'
+    = link_to t('ui.show'), finance_link_path(@invoice.financial_link)
 
 - if @invoice.user_can_edit?(current_user)
   = link_to t('ui.edit'), edit_finance_invoice_path(@invoice), class: 'btn'