Split base styles into base.css

That way we can load this css in other projects to bring the basic stuff
such as normalize, navbar, notifications, custom scrollbars, and so on.

src/styles/theatre.sass

@@ -1,11 +1,5 @@
-@import _normalize
-@import _config
-@import _utils
-
-@import _base
+@import base
 @import _comments
-@import _notifications
-@import plugins/js_perfectscrollbar
 
 $color-theatre-background: #222
 $color-theatre-background-light: lighten($color-theatre-background, 5%)