refactor(scripts): use parent to fetch config in entrypoint

[RomanYarik]

Currently each iframe environment fetches the config for itself.

We've seen that if an iframe wishes to load from a source which is not in the same base host as the parent, it cannot fetch the configurations properly.

Using parent window for 2 purpuses:

1. fetch the configuration for each new child environment
2. cache results, so we will avoid unneeded cache

Ideally, this should be solved using some middle service which manages that for all the browser environments (maybe a service worker), but it requires extra research

[tomermes]

Solved the issue with the full app (1 out of 3 "replaces")