Move import of excerpt CSS into app.scss

https://github.com/hypothesis/h/pull/2451#discussion_r41639447

h/static/styles/app.scss

@@ -8,6 +8,7 @@ $base-line-height: 20px;
 @import './common';
 
 // components
+@import './excerpt';
 @import './primary-action-btn';
 @import './dropdown-menu-btn';
 @import './publish-annotation-btn';

h/static/styles/common.scss

@@ -5,7 +5,6 @@
 @import 'mixins/responsive';
 @import 'grid';
 @import 'annotations';
-@import 'excerpt';
 @import 'forms';
 @import 'markdown-editor';
 @import 'spinner';