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.go

@@ -833,6 +833,7 @@ func (p *Page) Menus() PageMenus {
 					menuEntry.marshallMap(ime)
 				}
 				p.pageMenus[name] = &menuEntry
+
 			}
 		}
 	})