resources/page: Re-introduce .Page.Page

It was removed in 597e418cb0 -- it's not documented, but it's used in too many real sites on the web.

See #5784

resources/page/testhelpers_test.go

@@ -360,6 +360,10 @@ func (p *testPage) Params() map[string]interface{} {
 	return p.params
 }
 
+func (p *testPage) Page() Page {
+	return p
+}
+
 func (p *testPage) Parent() Page {
 	panic("not implemented")
 }