Header redirect to GOV.UK

[BenMillar-MOJ]

What does this pull request do?

• The header URL and "/" endpoint redirects to the GOV.UK service start page on production, and clears the users session
• The users session is no longer cleared by the category landing page
• The GOV.UK link in the header directs to the GOV.UK homepage
• The category landing page now exists at /find-your-problem as intended

Any other changes that would benefit highlighting?

Intentionally left blank.

Checklist

• Provided JIRA ticket number in the title, e.g. "LGA-152: Sample title"

[TawneeOwl]

LGTM