Don’t name SASS files with underscores

Underscores at the start of filenames are a convention that’s supposed
to tell a preprocessor to ignore a file. Since we’re explicitly
specifying which files need to be preprocessed, this isn’t necessary.

Also, it makes SASS lint throw a warning.

app/assets/stylesheets/main.scss

@@ -47,7 +47,7 @@ $path: '/static/images/';
 @import 'components/browse-list';
 @import 'components/email-message';
 @import 'components/api-key';
-@import "components/_previous-next-navigation";
+@import 'components/previous-next-navigation';
 
 @import 'views/job';
 @import 'views/edit-template';