Fixing warnings on User Fields settings page

[dparker1005]

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Changes proposed in this Pull Request:

pmpro_get_field_html() and pmpro_get_field_group_html() echo their contents immediately. Those functions do not return their contents.

This PR fixes warnings with code that expects the contents to be returned instead of already being echoed.

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Changelog entry

Enter a summary of all changes on this Pull Request. This will appear in the changelog if accepted.

[andrewlimaza]

This looks good! Making an internal note that we do escape and sanitize within those callbacks already.