Interface should be clearer that you can't delete your proofs

Being a dope, I assumed I could safely delete proof after it was confirmed by keybase. You might want to put a message on the dialog where proof is set up to make that clearer.

Also, I'm still unclear about _why_ proof has to remain. My proof tweet remaining on Twitter (for example) doesn't make the account any more **mine**. It makes a little sense for a website (shows that nobody else has taken ownership of the domain and replaced the proof content) but that's only really incidental, and doesn't stop somebody maliciously pretending to be me. It's seems to me that it's only meaningful to have the proof remain permanently if it's routinely updated.

Start by locating the proof-setup dialog in the Keybase client or website and inspect how its current messaging is presented. Update the interface so users understand that confirmed proofs cannot be deleted, and address the requested explanation of why proofs must remain; done means the dialog communicates both points clearly.

Written by the indexing model from the issue text.