`XH.clientAppCode` should be defaulted by dev-utils

[amcclain]

Currently clientAppCode can be set via AppSpec to any value, and is defaulted to the overall project-level appCode. Unfortunately our new BaseOauthClient expects clientAppCode to match the base route of the app - e.g. admin for the admin console app:

https://github.com/xh/hoist-react/blob/1bec2c2b748a9b684e8511cd6c4479c41bf3fd5b/security/BaseOAuthClient.ts#L231

This means that, for those baseUrls to work, devs must always spec a code that matches the name of the bundle entry-point file under client-app/apps/.

We should update hoist-dev-utils to define an xhClientAppCode magic const like the others:

https://github.com/xh/hoist-react/blob/1bec2c2b748a9b684e8511cd6c4479c41bf3fd5b/core/XH.ts#L101

and set it to the bundle entry-point. We could discuss if we need to support overriding that code, but I don't think we should. One important note however is that these client app codes are used by some other services such as alert banner presets and auto-refresh + idle configs. We should have a warning in the changelog to indicate that these should be reviewed and adjusted if an app code was previously spec'd that differs from the bundle name.