Provide a way to know if hm-admin is ready

[gorghoa]

Prerequisites

• Are you running the latest version of your current branch?
• Did you check your issue is not already reported?
• Have you tried to poke someone on Gitter?

Description

Since CoreDefinitionService::loadDefinitions() and SchemaService::loadSchema() don’t return observables (they return undefined), there is no way to know if hmadmin is ready or not.

Hence, I can’t add a pre-loader to my components waiting for hm-admin to do its initialization black magic.

I would love to have, as @Gregcop1 suggest, an “appReady” observable/promise