Guest Contributors - Enhancements

After the main PR ( https://github.com/Automattic/newspack-migration-tools/pull/49 ) is merged, do this:

- Check for email addresses in display name so that user_login, user_nicename, nickname, etc, don't end up being "myname_gmail_com" or something that could be used to hack the account.

- Keep the GuestContributorsHelper class, but move much of the underlying functionality into the UsersHelper class. The GuestContributors would just call the UsersHelper with the guest contributors role.

- Create an "assign to posts" function. ([This is currently](https://github.com/Automattic/newspack-migration-tools/blob/deeefd47fd486820b5e6c9a72234cfb0d4cf22e5/docs/guest-contributors.md#to-assign-guest-contributors-to-posts-use-coauthors-plus) in the help doc, but make it an actual function).

- Make a single `get_create_contributor` function.

- Related: https://github.com/Automattic/newspack-migration-tools/issues/41

- Trim `display_name` to 250 chars during data validation per this comment thread: https://github.com/Automattic/newspack-migration-tools/pull/119#issuecomment-3472266489 [PR](https://github.com/Automattic/newspack-migration-tools/pull/125)

Start with docs/guest-contributors.md and the GuestContributorsHelper and UsersHelper classes after PR 49 is merged. Review related issue 41 and the discussions in PRs 119 and 125, then define the work across contributor creation, display-name validation, post assignment, and helper responsibilities. Done means each listed enhancement is implemented and the documentation reflects the actual function.

Written by the indexing model from the issue text.