Add config option to define base URLs for GraphQL view queries

[el-schneider]

Querying e.g. seo > view > head uses the root URL of the statamic application for generating canonical links, which often doesn't make sense for a headless app. The favicon on the other hand seems to be a relative URL, which also breaks on a headless setup. I think it would be nice if there could be config options for app_url or frontend_url and asset_url, although Statamic only seems to be using the corresponding ENV vars APP_URL and ASSET_URL for CLI tools.

[aerni]

I believe there was a reason I didn't add this feature. But I can't quite remember why. I will look into it.