Studio: Refactoring font-weight CSS rules to use Sass placeholders

This clean-up work refactors font-weight CSS rules to use Sass placeholders. **It is meant for better FED abstraction/mgmt and should not have any significant user facing impact**.

A few notes:

* any Sass rules affected have also had their property order re-organized to have ```@include *```, ```@extend *```, and then all other rules
* any previously set font-weight of 500 has been set to use 600 (```%t-strong```) instead for simplicity and minimal difference.
* further refactoring is definitely needed after reviewing how many font-weights have set on similar or like-minded UI elements throughout the past couple of years.
This reorder is changing what will take precedence. Move the ui-btn-flat-outline up above the specific font setting. Also, the ui-btn-flat-outline already has t-strong as part of its definition, so we can just drop it here.