Avoid SCSS divisions outside calc expressions

Summary:
As Dart Sass deprecates the use of / as a divide symbol,
best course of action is to substitute them by a multiplication.

Reference: https://sass-lang.com/documentation/breaking-changes/slash-div

Test Plan: npm start

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D2927