Add role_invoices

This new role allows users to create invoices without role_finance. Users
can then only modify their own created invoices until somebody with the
role_finance sets the paid_on value.

app/views/workgroups/edit.html.haml

@@ -41,6 +41,10 @@
     = f.label :role_finance
     %br/
     = f.check_box :role_finance
+  %p
+    = f.label :role_invoices
+    %br/
+    = f.check_box :role_invoices
   %p
     = f.label :role_orders
     %br/

app/views/workgroups/index.html.haml

@@ -11,6 +11,7 @@
     %th Role Suppliers
     %th Role Article Meta
     %th Role Finance
+    %th Role Invoices
     %th Role Orders
     %th Deleted At
     %th Contact Person
@@ -28,6 +29,7 @@
       %td= h workgroup.role_suppliers
       %td= h workgroup.role_article_meta
       %td= h workgroup.role_finance
+      %td= h workgroup.role_invoices
       %td= h workgroup.role_orders
       %td= h workgroup.deleted_at
       %td= h workgroup.contact_person