Add language translation to opencdms-api

[isedwards]

• Investigate how pygeoapi implements language translation (using Babel). Specifically - whether/how field names from the model are translated and whether returned data is translated.
• Can we integrate the solution with a crowd sourced translation service like CrowdIn.
• The API should allow a logged in user to select a language (initial choices are English en-GB and French fr-FR)
• For convenience, the API should return the language code for the currently logged in user within the JWT token (also move first_name and last_name into the JWT token so that the data persists between requests).