Optimize the multilanguage build process

Work In Progress!

This commit makes a rework of the build and rebuild process to better suit a multi-site setup.

This also includes a complete overhaul of the site tests. Previous these were a messy mix that
were testing just small parts of the build chain, some of it testing code-paths not even used in
"real life". Now all tests that depends on a built site follows the same and real production code path.

See #2309
Closes #2211
Closes #477
Closes #1744

hugolib/page_permalink_test.go

@@ -23,8 +23,7 @@ import (
 )
 
 func TestPermalink(t *testing.T) {
-	viper.Reset()
-	defer viper.Reset()
+	testCommonResetState()
 
 	tests := []struct {
 		file         string