De-duplicate Doks' customized Bootstrap SCSS variables from default ones

[salim-b]

Summary

Doks heavily customizes Bootstrap SCSS variables (which is fine, of course). But when developing Doks inital styling, the whole set of Bootstrap variables was basically duplicated (only the !default modifiers were removed).

Ideally, only the SCSS rules that deviate from Bootstrap's defaults should be included in Doks' _variables.scss and _variables-dark.scss.

Bootstrap's default _variables.scss and _variables-dark.scss are already loaded by app.scss, so we can clean up Doks custom variables without breaking the site.

Motivation

Maintainability.