Atlas Core: Import App custom-variables before the module own variables

[askii2]

With this change, variables values set in theme/web/custom-variables.scss using the !default flag
would take precedence over the values in themesource/atlas_core/web/_variables.scss like the normal assignments.

Our use case

We use two UI Modules:

1. Our Style System, created using the tutorial in the docs, which we use in all our projects.
2. Branding modules for some of our clients, containing only their colors and logo images.

The 1^st sets default values for the $brand- variables, which the 2^nd overrides when imported first.
However when the 2^nd module is not present, the values in the 1^st are ignored because of the !default flag.