Default API and MongoDB endpoints to localhost

[erdnaxe]

First of all, thank you for sharing this awesome code!

This pull request proposes to default all API and MongoDB endpoints to localhost when the respective environment variable is absent. This seems to be a sanier default than hardcoding containers hostname in the code. It also renames REACT_APP_FLAG_REGEX to FLAG_REGEX, which might have been a bug.

Best regards,

[erdnaxe]

[RickdeJager]

Thanks!