Centralize UI endpoint configuration

crudOperations.js contains a hardcoded URL for instantiating axios.

[x] Externalize it in central configuration using .env file. You can see ndp-ui for example.
[x] Add how to configure this in README
[x] axios is not used, but instead fetch() is. This is not a problem, but remove the former if it will not be used

igrigorov / spring-react-i18n-admin