frontend: handle config overrides _after_ config server is up

Created by: mrnugget

This fixes #4123, or at least attempts to.

When DEV_OVERRIDE_EXTSVC_CONFIG or EXTSVC_CONFIG_FILE are set, handleConfigOverrides calls db.ExternalServices.Create, which in turn calls conf.Get(), which waits for the configuration server to have booted up.

But with 507371f2 this call to handleConfigOverrides was now made before the configuration server has booted up, with the result that the whole frontend process ran into a deadlock since every goroutine was waiting for the conf server to start up, including the goroutine responsible to do so.

This commit does not move the handleConfigOverrides call to its original place but instead in between the configuration-server-start-up and the validation of the configuration.

I assume that this was the intention behind moving handleConfigOverrides() in the first place: the overrides should take place before the defaults are validated.

Test plan: ./dev/launch.sh and ./enterprise/dev/start.sh (one sets the ENV vars and one does not)