chore: imrpove guest author migration

[leogermani]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

A few more tweaks to handle edge cases:

• Handle user nicenames that exceeds the allowed size (shorten them and make sure to redirect the old slug)
• Handle email max size
• By default, just assign the existing images as the user avatar, instead of sideloading and creating a new image in the media library for each avatar.

How to test the changes in this Pull Request:

Tested

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[jason10lee]

Debugged in huddle with @leogermani; fix tested and committed. Good to go!

[github-actions]

Hey @leogermani, good job getting this PR merged! 🎉

Now, the needs-changelog label has been added to it.

Please check if this PR needs to be included in the "Upcoming Changes" and "Release Notes" doc. If it doesn't, simply remove the label.

If it does, please add an entry to our shared document, with screenshots and testing instructions if applicable, then remove the label.

Thank you! ❤️