Bump laravel-mix and laravel-mix-purgecss

Bumps [laravel-mix](https://github.com/JeffreyWay/laravel-mix) and [laravel-mix-purgecss](https://github.com/spatie/laravel-mix-purgecss). These dependencies needed to be updated together.

Updates `laravel-mix` from 2.1.14 to 4.0.12
- [Release notes](https://github.com/JeffreyWay/laravel-mix/releases)
- [Commits](https://github.com/JeffreyWay/laravel-mix/compare/v2.1.14...v4.0.12)

Updates `laravel-mix-purgecss` from 3.0.0 to 4.0.0
- [Release notes](https://github.com/spatie/laravel-mix-purgecss/releases)
- [Changelog](https://github.com/spatie/laravel-mix-purgecss/blob/master/CHANGELOG.md)
- [Commits](https://github.com/spatie/laravel-mix-purgecss/compare/3.0.0...4.0.0)

Signed-off-by: dependabot[bot] <support@dependabot.com>

package.json

@@ -25,8 +25,8 @@
     "jquery-minicolors": "^2.1.10",
     "jquery-serializeobject": "^1.0.0",
     "jquery-sparkline": "^2.4.0",
-    "laravel-mix": "^2.1",
-    "laravel-mix-purgecss": "^3.0.0",
+    "laravel-mix": "^4.0",
+    "laravel-mix-purgecss": "^4.0.0",
     "livestamp": "git+https://github.com/mattbradley/livestampjs.git#develop",
     "lodash": "^4.17.11",
     "messenger": "git+https://github.com/HubSpot/messenger.git",