Update dependencies

Summary:
gulp-imagemin 8.0.0 is blocked by T1660, 7.1.0 is fine.
jpegtran isn't maintained, use mozjpeg instead.
Add presets for GIF, JPG and PNG.

Use dart sass as SCSS compiler.

Ref T1654

Test Plan: Build test site from template

Reviewers: dereckson

Reviewed By: dereckson

Maniphest Tasks: T1654

Differential Revision: https://devcentral.nasqueron.org/D2432