GPG/redesign

[gdixon]

Description

EPIC - new brand styles integration

This PR:

• Introduces django-libsass
• Contains a test stylesheet (./app/assets/v3/scss/main.scss) to check that libsass will work in our environment
• Replaces bootstrap.min.css with bootstrap.scss
• Replaces bootstrap-vue.min.css with bootstrap-vue/index.scss
• Replaces vue-select.css with vue-select.scss
• Adds minification rules for production env
• Moves libs to v3/libs
• Updates bootstrap v4.3.1 -> v4.6.0
• Updates bootstrap.min.js v4.3.1 -> v4.6.0
• Updates popper.min.js v1.16.0 -> v1.16.1
• Moves v3/scss assets into v2/scss - fixing problems with collectstatic
• Updates and internalises jQuery (v3.3.1 -> v3.5.1)
• Fixes google tagmanager on landing
• Implements new scss libs on landing
• Moves libs back in to "lib" dir (./app/assets/v2/scss/lib/*)

Refers/Fixes

Fixes: #8363
Refers: #8308
Refers: #8358

[amustapha]

This is massive and can barely find something wrong with it. I'll give it a pass.