Support certificate exchange via invitation tokens, distributed over Twitter

This pull request is part of a long set of changes allowing certificate exchange (between two personal zone hubs) to be performed using invitation tokens.  

Specifically, it means that Alice can send an invitation link to Bob, who can then click on it to connect the two PZHs.

Unfortunately the URL being generated is currently invalid.  But this does add support for:
 * Twitter DM distribution
 * Invitation tokens

It also improves stability of all certificate exchange processes, simplifying the protocols.

Related, dependent PR: https://github.com/webinos/webinos-pzh/pull/13

Jira issue: WP-1064 
Jira issue: WP-852