Upgrade GOV.UK Frontend to 5.3.0

[markhobson]

GOV.UK Frontend should be upgraded from 5.1.0 to 5.3.0.

[tim-s-ccs]

I've had a look at doing this in my fork of this repo https://github.com/tim-s-ccs/govuk-frontend-jinja/pull/7

If my changes regarding testing are accepted then this can obviously be use to help speed things along

[matthew-shaw]

Updated in #78 👍🏻

[markhobson]

Thanks @matthew-shaw!