Bump govuk_frontend_toolkit to 5.0.0

This release includes two breaking changes:

Removal of external link styles and icons, if you are using the
external-link-* mixins you will need to remove them from your codebase
(PR #293)
Correct spelling of the 'accordion' icon, you will need to check for
the incorrect spelling 'accordian' and update if you are using this
icon (PR #345)
And two minor changes:

Amend GOVUK.StickAtTopWhenScrolling to resize the sticky element and
shim when the .js-sticky-resize class is set (PR #343)
Allow custom options in GOVUK.analytics.trackPageview (#332)

package.json

@@ -21,7 +21,7 @@
     "express-session": "^1.13.0",
     "express-writer": "0.0.4",
     "govuk-elements-sass": "2.0.0",
-    "govuk_frontend_toolkit": "^4.18.4",
+    "govuk_frontend_toolkit": "^5.0.0",
     "govuk_template_jinja": "0.18.3",
     "gulp": "^3.9.1",
     "gulp-clean": "^0.3.2",