Update Twitter verification tweets with App.co URL

[markmhendrickson]

Currently all Twitter verification tweets include a link to https://blockstack.org/try-blockstack, even though that resource not permanently redirects to App.co.

As a consequence, Fathom is not tracking this inbound traffic by referrer, for either domain. We should update these tweets to include a link to http://app.co/ directly instead of https://blockstack.org/try-blockstack so that Fathom attributes this traffic to Twitter effectively and we can determine accurately just how much Twitter proofs are driving in-bound interest.

@CharlieC3 @cuevasm @wileyj I believe this change is necessary to get a proper read on this traffic before we can make any decision as to whether we want to keep Twitter proofs or not in the legacy Blockstack Browser. Please let me know if you have any other ideas though.