Add new ResolveMany mutation for todos

What does this MR do and why?

This adds a new ResolveMany graphql mutation for todos.

Because this shares almost all of its logic with the existing RestoreMany mutation, I refactored it into a shared base clase (Template pattern).

In later MRs, we will add also add mutations for SnoozeMany and UnsnoozeMany.

References

• Belongs to issue Bulk edit to-dos (#16564 - closed) • Paul Gascou-Vaillancourt, Thomas Hutterer • 17.10 • Needs attention
• Prettier version of my copy&pasta in Draft: Resolve "Bulk edit to-dos" (!178323 - closed) • Thomas Hutterer • 17.10 🍝

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

From http://gdk.test:3000/-/graphql-explorer:

How to set up and validate locally

• Make sure this doesn't break our existing To-Do dashboard feature: http://gdk.test:3000/dashboard/todos
  • The existing, refactored RestoreMany is used when you click Undo (in the toast at the bottom-left) after clicking the Mark all as done
  • So to verify, Mark all as done, then Undo, and your todos should come back.
• Test the new ResolveMany in the GraphQL explorer: http://gdk.test:3000/-/graphql-explorer