[Enhancement]: Allow option for custom logo

### What should change?

This would allow services to have a custom logo set up when they are unable or do not want to use the GO.UK logo.

### User need

This is a recommended pattern in the [service manual](https://www.gov.uk/service-manual/design/making-your-service-look-like-govuk#if-your-service-isnt-on-govuk).

### Proposed solution

see linked PR #435 

also addresses #138 