update header to allow homepage only and add to the login page

[cjrace]

Overview of changes

Quick one to add a button to go back to the homepage from the login page as I realised we didn't have one and I got stuck on the login page on our tablet.

Edited the existing header function to add in a homeOnly option that allows for this and added a quick test for it too.

Checklist

• I have tested these changes locally using pnpm test
• I have updated the documentation (if needed)
• I have added or updated tests for these changes (if applicable)

Reviewer instructions

Not a lot to test here, though this is what it looks like (before there was nothing in the top left of the page)