Multiple commits:
* Remove View::share('fluent_layout') and instead detect if Orchestra\Site::has("layout.fixed") to determine layout structure.
* Remove `orchestra.backend: bottom` asset container and replace with `orchestra.backend: footer`.
* split `orchestra::layout.main` to have `orchestra::layout.header` and `orchestra::layout.footer` to help templating.

Signed-off-by: crynobone <crynobone@gmail.com>