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/project-main.sass

@@ -8,10 +8,7 @@
 @import _project-dashboard
 @import _error
 
-@import _base
-@import _notifications
 @import _search
 
 @import plugins/_jstree
-@import plugins/js_perfectscrollbar
 @import plugins/_js_select2