Allow several custom stylesheets

So other institutions customizing CONSUL can organize their custom code
as they wish to.

We're also updating the comments in the `custom.scss` file, since it was
referencing files which no longer exist.

app/assets/stylesheets/application.scss

@@ -39,3 +39,4 @@
 @import "widgets/**/*";
 
 @import "custom";
+@import "custom/**/*";

app/assets/stylesheets/custom.scss

@@ -1,5 +1,4 @@
-// Overrides and adds customized styles in this file
+// Override and add customized styles in this file.
-// Read more on documentation:
-// * English: https://github.com/consul/consul/blob/master/CUSTOMIZE_EN.md#css
-// * Spanish: https://github.com/consul/consul/blob/master/CUSTOMIZE_ES.md#css
 //
+// You can also create SCSS files in the app/assets/stylesheets/custom/
+// folder and they will be loaded automatically.