Add locale to projects table

[IzzySmillie]

• Story: https://github.com/RaspberryPiFoundation/digital-editor/issues/75
• As mentioned in both ADRs:
• • E2E
• • Intial metrics

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• Add new column to the projects table for the project locale
• Make sure it populates en for existing starter projects and null for custom projects

[create-issue-branch]

Branch issues/140-Add_locale_to_projects_table created!