Upgrade Bourbon to 5.1.0 and fix deprecation warnings (#1973)

- Replace uses of deprecated Bourbon helpers with raw CSS
- Add autoprefixer into the build chain to prefix the now removed helpers

This process was achieved by running through each of the deprecation warnings and using the following bash function to replace it in all files:

```
function bourbon-deprecate() {
    grep -rl "@include $1" ./frontend --exclude-dir=.git | xargs sed -i '' -E "s/@include $1[(](.*)[)]/$1: \1/g"
}
```

For some helpers, this did not result in valid CSS, so manual modifications were made.

Closes #1189 #1274

frontend/components/forms/ChangeEmailForm/_styles.scss

@@ -1,7 +1,7 @@
 .change-email-form {
   &__btn-wrap {
-    @include display(flex);
-    @include flex-direction(row-reverse);
+    display: flex;
+    flex-direction: row-reverse;
   }
 
   &__btn {