[gnome-shell-sass] theme: Replace gnome-shell-sass submodule with subtree

[Florian Müllner <fmuellner src gnome org>]

commit be0597d88d215044629664b6f9a74a475313d599
Author: Florian Müllner <fmuellner gnome org>
Date:   Fri Feb 9 19:22:55 2018 +0100

    theme: Replace gnome-shell-sass submodule with subtree
    
    As the style has grown bigger and more complex, generating the different
    variants from a common source has been a good decision. However given how
    intertwined the theme is with gnome-shell itself, relying on a submodule
    has proven to be quite painful. And as things stand right now, it is going
    to get worse:
    
     - using either pre-generated CSS or generating it at build time is
       odd, and violates meson's strict separation between source- and
       build directories; we are therefore considering dropping the CSS
       and depending on sassc to always generate it at build time
    
     - with the migration to gitlab, our workflow shifts decisively towards
       branches; however there is no support in either git or gitlab for
       handling two brances of separate repositories consecutively, which
       gets particularly awkward for branches in a private namespace
    
    With those pain points in mind, we will adjust our setup as follows:
    
     - remove the submodule from gnome-shell and instead import the
       sass as subtree
    
     - after that, the sass sources can be changed like any other files
       in the repository, and regular contributors can forget that there
       was ever anything special about them
    
     - whenever we want to update the classic style, we can push the subtree
       changes and bump gnome-shell-extension's sass submodule
    
    In other words: Updating the classic styling will become slightly more
    painful, but not much and only for me; in return, everyone else can
    stop fiddling with submodules (and buy me a beer).