Extend the demo with deploying Locales to demonstrate multi-country and multi-currency

Problem/Motivation

We should extend the demo with deploying Locales to demonstrate multi-country and multi-currency features of the module.

Proposed resolution

1. Extend the demo accounts yaml with the list of supported locales.

2. On deploy - check if the current locales exist with the same id.
- if not exist - create the locale (eg "de") with the custom name of the country "Germany" instead of the language "German".
- if yes - create a locale with the "-ct" suffix and the custom name of the country ("de-ct": "Germany"), and configure the commercetools country-currency mapping for it manually.

And store in the state which locale we created, to remove it on uninstall.

3. Add the language switcher block to the site header. If Drupal Core provides no such block, just choose the simplest one from contrib (starting from here https://www.drupal.org/project/dropdown_language → ) and copy-paste it to the demo module (to not bring in additional dependency).

4. Rework the current description on the /admin/config/system/commercetools/store to make it more visible, design it as the info message, with "Dismiss" option to hide it permanently (save the status in the Drupal State).