Transparent accounting for organizations

[Changaco]

In addition to #482 we could implement the posting and settlement of invoices, similar to Open Collective. We need/want this for the Liberapay org itself (see liberapay/liberapay.org#20 (comment)), so we might as well build a tool that other organizations can use too. This requires changing our Terms of Service to allow other kinds of payments than donations.

[Changaco]

I promised Zatalyz she'll be reimbursed in the next few weeks for liberapay/salon#100 so I'm starting the work on this.

[Changaco]

I've made some progress on this but I'm a little stuck on the storage issue. It seems to me that the correct solution would be to use S3, but if we start using AWS then we should use it for everything in order to fully benefit from the 1-year free tier (see #177 (comment)).

[Changaco]

[Changaco]

I've enabled invoices for LiberapayOrg, @Zatalyz, and myself.

(update participants set allow_invoices = true where username in ('LiberapayOrg', 'Zatalyz', 'Changaco'))

[Changaco]

And here's our very first invoice: https://liberapay.com/LiberapayOrg/invoices/1 🎉

Next step is implementing the other side (acceptance and payment of the invoice).

[Changaco]

Our first two invoices are now paid! 💃 https://liberapay.com/LiberapayOrg/invoices/