fixes #8794

Reported the underlying issue here:
https://github.com/rails/sass-rails/issues/406

Basically `@import` works like this:
* look for the file relative to the current file
* look for the file as an absolute path following the priorities in the
* asset load_paths

If the file can be imported as a relative path that will take
precedence.

So in order to pick up the head and tails inside customization rather
than in app/assets there are three possibilities:
1) use an absolute path. This is not as easy as it seems. There is no
way of indicating a path is meant to be absolute so we would have to
ensure it does not resolve to a relative path.
2) have a application.scss file inside the customization folder. Since
this is the main file it will be used instead of the app/assets one. In
there relative paths will now also default to the customization folder
rather than app/assets. Once we are in an app/assets file though it will
not go back to picking up customization with relative paths
3) use //= require instead of import. rails-sass advices against this as
each required file would be compiled on it's own and variables could not
be shared.

Going with option 1 here:
```scss
// application.scss:
@import "custom/head_import";
```

```scss
// custom/head_import.scss:
@import "head";
```

As long as there is no custom/head.scss in app/assets it will import
head as an absolute path and thus prefer config/custom over app/assets.

This seems like the best option for now as it does not require changes
to the deployments.