Add maintenance mode

[bodhish]

Proposed Changes

@pupilfirst/developers

Merge Checklist

• Add specs that demonstrate bug / test a new feature.
• Check if route, query, or mutation authorization looks correct.
  • Add tests for authorization, if required.
• Ensure that UI text is kept in I18n files.
• Update developer and product docs, where applicable.
• Prep screenshot or demo video for changelog entry, and attach it to issue.
• Check if new tables or columns that have been added need to be handled in the following services:
  • Users::DeleteAccountService
  • Courses::CloneService
  • Courses::DeleteService
  • Courses::DemoContentService
  • Levels::CloneService
  • Schools::DeleteService
• Check if changes in packaged components have been published to npm.
• Add development seeds for new tables.
• If the updates involve Graph mutations ensure that the files are migrated to the new approach without a mutator.

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[yash-learner]

LGTM 👍