Replace gulp-base64 with gulp-base64-inline

The gulp-base64 package has 11 dependencies with
vulnerabilities listed against them as of this
time. It also doesn't seem to be maintained any
more. The last commit was in 2015 and there are
issues and pull requests up to bump the
dependencies.

This replaces it with gulp-base64-inline.

gulp-base64-inline takes a single path, which it
prepends to any image paths it finds. Our image
paths are actually URLs, not filesystem paths so
we need to send it a relative path to repoint the
URL.

This commit includes changes that remove a few
`@import`s from one of our sass partials.

They aren't needed as those files are imported
further up the stack and `_typography.scss` has an
import in it that overwrites the new
`_url-helpers.scss` we added here.

package.json

@@ -27,7 +27,7 @@
     "gulp": "4.0.0",
     "gulp-add-src": "1.0.0",
     "gulp-babel": "8.0.0",
-    "gulp-base64": "0.1.3",
+    "gulp-base64-inline": "1.0.4",
     "gulp-concat": "2.6.1",
     "gulp-include": "2.3.1",
     "gulp-load-plugins": "1.5.0",