VaultPress Dashboard: Mention how to get the VP key

I think we can do a better job helping folks to find their registration key. Having a link to their VP dashboard right there might simplify the registration process (actually, I not really sure what is the desired way to get VP key)

The issue provides no file, test, or entry point. First locate the VaultPress registration-key UI and confirm the intended dashboard link and key-retrieval flow; done means the registration screen clearly explains how to obtain the key and the link works.

Written by the indexing model from the issue text.