Added user preference for work items view

What does this MR do and why?

This MR adds a user preference. There is no frontend available to toggle it yet, but will eventually allow the user to view issues as work items instead of legacy issues. See #461855 (closed) for more details.

This also exposes user preferences in the GraphQL API, which was not previously available.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

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Before	After

How to set up and validate locally

GraphQL query and mutation are part of this MR, so you can use GraphiQL to validate this change:

1. Go to http://127.0.0.1:3000/-/graphql-explorer
2. Add the following:

query getUserPreferences {
  currentUser {
    userPreferences {
      useWorkItemsView
    }
  }
}

mutation updateWorkItemView {
  userPreferencesUpdate(input: {useWorkItemsView: true}) {
    userPreferences {
      useWorkItemsView
    }
  }
}

3. Run getUserPreferences, should be false. Run updateWorkItemView, then run getUserPreferences again and it should now be true.