Improved email lookup with seon fallback

[SanderGi]

Task: https://app.asana.com/0/1203067047205953/1205752833679516

Split off from: #206

Q/A checklist

• Do a code review of the changes
• Add any new dependencies to poetry & export to requirementst.txt (poetry export -o requirements.txt)
• Carefully think about the stuff that might break because of this change
• The relevant pages still run when you press submit
• If you added new settings / knobs, the values get saved if you save it on the UI
• The API for those pages still work (API tab)
• The public API interface doesn't change if you didn't want it to (check API tab > docs page)
• Do your UI changes (if applicable) look acceptable on mobile?

[devxpy]

seon no longer affordable