switch to using Gittip's Venmo app

[chadwhitacre]

In the interest of time, we launched Venmo integration using an oauth app registered under my personal Venmo account (when will I ever learn 😢). We've since been approved for a business account, but I'm hitting a snag trying to register an app under that account:

[chadwhitacre]

This is hot right now because we need to tell users something about gratipay/security-00000#1 and if we have to have everyone who's ever connected a Venmo account reconnect their account it'd be convenient to announce that as part of gratipay/security-00000#1.

[chadwhitacre]

From @simon-weber in private IRC:

apparently the "you may already have an account" message is because the same identity (last 4, etc) is linked to the personal account and the business account let me find out what we can do about it

And then in logged IRC:

whit537: I pinged the compliance person you originally got the bad news from spoke to to see how we normally deal with two accounts linked to the same identity it seems like it's totally reasonable thing to happen, so I'm sure we've seen it before

[chadwhitacre]

Thanks @rohitpaulk. :-)

Fixed at #2810 (comment).