refactor: convert CSS to SCSS

Remove Bootstrap from Header and assets.toml
  Remove Bootstrap JS CDN version in footer and assets.toml
  Add latest Bootstrap v4 SASS files locally
  Include Bootstrap JS locally
  Convert Academic's CSS to SASS (mainly SASS variables + Go templating)
  Convert Academic's theming system to integrate with SASS
  Parse SASS in header using Hugo and convert to CSS
  Minify and md5 fingerprint bundled CSS
  Fix Slider widget (reuses .wg-hero but animation and `clear` removed)
  Add Bootstrap files to repo so that Node is not required

See #680

package.json

@@ -0,0 +1,23 @@
+{
+  "name": "academic",
+  "version": "1.0.0",
+  "description": "The Page Builder for Hugo",
+  "main": "index.js",
+  "dependencies": {
+    "bootstrap": "^4.3.1"
+  },
+  "devDependencies": {},
+  "scripts": {
+    "test": "echo \"Error: no test specified\" && exit 1"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/gcushen/hugo-academic.git"
+  },
+  "author": "George Cushen",
+  "license": "MIT",
+  "bugs": {
+    "url": "https://github.com/gcushen/hugo-academic/issues"
+  },
+  "homepage": "https://github.com/gcushen/hugo-academic#readme"
+}