Link venmo

cc @simon-weber 

Issue: https://github.com/gittip/www.gittip.com/issues/1839

This PR adds [Venmo](https://venmo.com) as a linkable account (but *not* an auth-able one). This is the first step towards several possible new features powered by Venmo (using Venmo as a funding source for recurring tips, using Venmo for paying out developers, and possibly using Venmo as a solution for #5).

This PR also does stopgap refactoring of several parts of elsewhere:

* Adds the ability to iterate over types of accounts with the new `platform_classes` registry
* Cuts down on copy-pasted template code using above
* Elsewhere records can now be used to construct `AccountElsewhere` objects so that convenience methods can be used on existing accounts
* Reorganized env variables file