Check #email_address for sidebar name

[tumes]

Description

Rails 8's new baked in authentication implementation uses user#email_address by default, so I figured it would be nice to support that by default in Avo as well to make things feel seamless for the new release. This is a pretty lightweight PR but please let me know if you've like a section in the docs specific to supporting the new baked in authentication since the manual testing steps below are juuuuust fiddly enough to maybe merit their own section.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works

Screenshots & recording

Manual review steps

1. Create a new app with the Rails 8 beta
2. Add authentication with bin/rails generate authentication
3. Create a user
4. Install Avo
5. Modify the Avo initializer to use Current.user for the current_user_method and session_path for the sign_out_path_name.
6. Modify the Avo initializer to use Rails' default authentication concern with:

Rails.configuration.to_prepare do
  Avo::ApplicationController.include Authentication
end

7. Append main_app. to any redirects in the Authentication concern.

Manual reviewer: please leave a comment with output from the test if that's the case.

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[adrianthedev]

I love this!
Thanks for being so proactive and sending this PR in ❤️