[BUG #6645] migrating .less to .scss

- flattening namespaced mixins
[BUG #6645] migrating .less to .scss

- reproducing base.css from base.scss (modulo formatting)
[BUG #6645] migrating .less to .scss

- reproducing layout.css from layout.scss (modulo formatting)
[BUG #6645] migrating .less to .scss

- reproducing pages.css from pages.scss (modulo formatting); incomplete
[BUG #6645] migrating .less to .scss

- reproducing pages.css from pages.scss (modulo formatting)
[BUG #6645] migrating .less to .scss

- reproducing reset.css from reset.scss (modulo formatting)

For this migration step, generated .css files were additionally formatted by
hand, to ease diffing. This doesn't have to happen on further generation runs.