Improve login integration with other services

[anjackson]

Currently, if you access an ancillary service (Grafana, reports, etc.), then you get bounced to ACT to login, but you are not bounced back afterwards. It would be good to be able to do this:

https://www.webarchive.org.uk/act/login?redirectToUrl=/act/static/reports/prod/report.html

i.e. the NGINX auth system could point you to the login, but then ACT would take notice of the redirectToUrl and bounce you back.