Refactor updateUserSettings mutation to accept partial updates.

Right now the updateUserSettings mutation requires that you pass in the entire userSettings object in order to update a field Otherwise it will overwrite the unchanged fields. It should instead only update the fields that were passed in and preserve the rest.

This mutation should also return an object with the applied changes so we can make effective usage of the apollo cache. instead of calling refetchQueries.

https://github.com/evergreen-ci/spruce/blob/373f8d96995bb8763873c32682f6039691fd5575/src/pages/preferences/preferencesTabs/NewUITab.tsx

https://github.com/evergreen-ci/evergreen/blob/d6889a75eacbfad076d95b00cda3e40331242aaf/graphql/resolvers.go#L2754