Allow theming / branding of graphiql interface

I'm submitting a ...

* [ ] bug report
* [x] feature request
* [ ] question

PostGraphile version: 4.4.0

It would be nice to be able to easily do some lightweight theming of the graphiql interface. Primarily, this involves changing or removing the logo and title at the top, and changing or removing the links in the footer to Postgraphile documentation and examples.

I would be happy to have the footer contain a "Powered by Postgraphile" link, but the current links would confuse end users who would be looking for documentation about the product or site using graphiql rather than the underlying tool.

Another thing we want to be able to do is to pass in an authentication string so the user doesn't have to manually click the headers and paste it in. One easy way I can think of to do this is for the link to open graphiql include an anchor reference (i.e http://localhost:9500/graphiql#Bearer=eyaid.a123245.asdf8g ) and have the JS in graphiql look for that and put it in the header on init if it is found.