Move inline styles to a separate CSS files

[danielledeleo]

This PR is necessary so that inline styles for the main Hugo theme don't interfere with Mainframe styles.

Comment from assets/css/inline-overrides.css:

/*
 * These styles were previously defined inline in the theme as hacks. They
 * live here so we know where they came from.
 *
 * Where necessary, new classes were defined in their place so they don't 
 * interfere with the new theme CSS. (e.g. .override-sidebar-collapse)
 **/

[nginx-aoife]

Testing
Compared preview site to prod

• No significant change in layout ✅
• All features working as before ✅