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