Remove basscss dependency

[mejiaj]

Basscss is deprecated and the github repo is archived. We should remove this dependency. Let's use USWDS tokens, utilities, and functions instead.

See main.scss

plainlanguage.gov/assets/css/main.scss

Lines 10 to 13 in 030125b

	@import 'basscss-sass/scss/basscss';
	@import 'basscss-sass/scss/responsive-margin';
	@import 'basscss-sass/scss/responsive-padding';

For example, handling margins
https://github.com/basscss/basscss-sass/blob/master/scss/_responsive-margin.scss

Use USWDS tokens for spacing instead
https://designsystem.digital.gov/design-tokens/spacing-units/

[nick-mon1]

This will be closed out when site is migrated.