Allow extra CSS be added via the UI

[markconroy]

It might be a good idea for people to be able to add some arbitrary CSS to the theme via the UI - e.g. early days, before you have a developer, but want the theme to have a bit of council branding.

I understand we probably don't want people doing this for production sites, but having the facility during dev could be welcome.

[finnlewis]

This is proving controversial, see #859

We need to discuss more and think through the user needs and implications.

@willguv is conscious that there are other higher priority issues that we probably want to focus on before we dive into this deeper.

Meanwhile we can continue the discussion ...