Project lang on save or remix

[IzzySmillie]

Some of this might be covered already in the Update webhook to add locale to DB ticket

• Story: https://github.com/RaspberryPiFoundation/digital-editor/issues/75
• "for blank projects we don't need to worry about saving the translated UI language"
• Set null on save of a blank project or remix of a starter project. The data for which lang the project was remixed from will still be available and will be easier when serving the project to the user based on the URL/interface locale

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• For blank projects set lang as null
• On remix of a starter project, lang should be set as null

[IzzySmillie]

Closing as remix was done in #156
and blank projects are being set as null already