I made this URL dynamic before, here, but that didn't work because the "default" value in the site.schema.json file doesn't do what I thought it did. So @chrsmith did a quick fix in https://github.com/sourcegraph/sourcegraph/pull/62835, but then we lost the ability to make this dynamic.

So this PR makes it dynamic again, this time with a working fallback.

Alternatives considered:

I tried to do the same thing on the back end, but it didn't feel right.

1. I tried a transformation in computed.go:

// GetCodyProConfig returns the CodyProConfig for the current site config,
// with some handy defaults.
func GetCodyProConfig(siteConfig schema.SiteConfiguration) (c conftypes.CodyProConfig) {
	c.SamsBackendOrigin = "https://accounts.sourcegraph.com"
	c.SscBackendOrigin = "https://accounts.sourcegraph.com"
	c.SscBaseUrl = "https://accounts.sourcegraph.com/cody"
	c.StripePublishableKey = nil

	if siteConfig.Dotcom == nil || siteConfig.Dotcom.CodyProConfig == nil {
		return
	}

	codyProConfig := siteConfig.Dotcom.CodyProConfig
	if codyProConfig != nil {
		c.SamsBackendOrigin = codyProConfig.SamsBackendOrigin
		c.SscBackendOrigin = codyProConfig.SscBackendOrigin
		c.SscBaseUrl = codyProConfig.SscBaseUrl
		c.StripePublishableKey = &codyProConfig.StripePublishableKey
	}
	return
}

But having the defaults there was meh, plus I had to introduce the new type conftypes.CodyProConfig but nothing prevents another eng to just use the config directly, which might become confusing.

2. I thought about setting the defaults in makeFrontendCodyProConfig, which might be better, but then, on the front end, we already have a single location where we access the SSC base URL, and we already had the default there, so I just went with that.

Test plan

Here is a 1-min Loom where I test it with different settings and demo that it works as intended.