ref(filmstrip): move filmstrip styles to filmstrip folder

css/main.scss

 @import '404';
 @import 'policy';
 @import 'popover';
-@import 'filmstrip';
 @import 'filmstrip/filmstrip_toolbar';
+@import 'filmstrip/horizontal_filmstrip';
 @import 'filmstrip/vertical_filmstrip_overrides';
 @import 'unsupported-browser/main';
 @import 'modals/invite/add-people';