mgr/dashboard: Add I18N/L10N support to the backend to return localized error messages
This came up during the review of PR32032 - by moving the validation of the password complexity to the dashboard backend / REST API, the frontend would need to translate the messages returned in order to provide a consistent user experience. In order to avoid the duplication of strings and to also support localized error messages for users talking to the REST API directly, we concluded to make the REST API return localized strings, based on the HTTP header submitted by the API client (e.g. the dashboard). The translations could then be managed via gettext and the Transifex translation platform.
Note that this will only apply to messages generated by the Ceph dashboard backend directly; the REST API will still return a lot of other information in English only (e.g. the Ceph health state), as these messages originate from other parts of the Ceph codebase.