Remove/separate build-time configuration

Nobody has claimed this yet.

The GEPPETTO_CONFIGURATION file contains a mix of build-time, backend and runtime configuration.
This limits the reusage of prebuilt images: the full image needs to be rebuilt to set it as embeddable or to set the ssl websocket communication.

The way we load the GEPPETTO_CONFIGURATION atm is not really a library issue, but rather a webpack configuration of the application using it: https://github.com/openworm/geppetto-application/blob/master/GeppettoConfiguration.json

I propose to move the loading of the variables needed ar runtime to a json loading from a subfolder of the main js folder (build), so that we can eventually load that json file as a ConfigMap on Kubernetes.

The variables that I'm aware having some usage in the client are `useSsl`, `contextPath`, `embedded`, `embedderURL`. The loading of this json file to create the GEPPETTO_CONFIGURATION can be moved in the geppetto-client init https://github.com/MetaCell/geppetto-meta/blob/feature/175/geppetto.js/geppetto-client/src/GEPPETTO.js#L134

The rest is mostly related to the main html page functionality, which can be revised as a separate issue.

Start with the GEPPETTO_CONFIGURATION loading in geppetto-client/src/GEPPETTO.js around line 134 and compare it with the application webpack configuration in GeppettoConfiguration.json. Trace the uses of useSsl, contextPath, embedded, and embedderURL. Done means runtime values load from a JSON file under the built JavaScript folder while build-time configuration remains separate.

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