Refactor CSS assets for easier management

[Bodacious]

The CSS assets are not organised very neatly. We need to refactor these to make it easier to support custom branding.

• Move CSS assets back to Rails's asset pipeline
• Add support for LibC Sass (faster compilation)
• Refactor CSS file structure for maintainability
• Normalise CSS variables

[sjDCC]

haven't noticed any issues in display with branding so assume this is ok. this is just based on general visual checks as I have done overall UAT. @xsrust @briri Can you advise if there are specific things I should test?

[briri]

Anything that includes Javascript (e.g. questions load when you expand a section, table sorting, etc.) and the site's look and feel (CSS).

[sjDCC]

All looks good to me. Table sort and search works well and all accordians open. Only issue I spotted is that links open in same window. We used to have it force a new window to be opened. Will raise a ticket for that and close this one out