Added sorting information to configuration parameter values

[ricardogsilva]

This PR adds a sort_order property to configuration parameter values which can be modified in the admin.

This sort order is later used for building the parameter combinations that are useful for the frontend to build its menus, i.e. in the /api/v2/coverages/forecast-variable-combinations and /api/v2/coverages/historical-variable-combinations API endpoints.

This PR does not actually expose the sort order in the API endpoint that provides variable combinations, but rather already provides this output sorted correctly. This means that the frontend client would not need to worry about sorting the output of the API. Regardless, if needed, the sort_order property can be found in the api/v2/coverages/configuration-parameters API endpoint

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• fixes Implement a way to sort a configuration parameter's possible values manually #256