Improvements for phishing warning

We would like to warn the user when a new sender looks like a known contact. Looks like means:

1. Similar display name but different mail address
2. Typos in domains (e.g. https://github.com/elceef/dnstwist)
3. Same identifier but different domains

We can also check the DKIM state (see #244). We cn also discuss to implement other ideas, e.g. from security guidelines or other tools.

• Add more communication channels (call, IM etc.)
• Consider display name for phishing detection
• Consider similar display names, similar mail addresses(just for completeness)

This issue should be split in more handy issues.