Make vokomokum plugin work

[wvengen]

To make the vokomokum plugin work again fully with the current Foodsoft version, the following is needed:

• Currently login works with form parameters (req.set_form_data(data)) and charge-members works with a JSON body (req.content_type = 'application/json'; req.body = data.to_json). Adapt the members app or the Foodsoft vokomokum plugin to get both working.
• When the members system gives an error when settling an order, handle this gracefully (instead of showing a fully empty screen right now).
• Get wvengen/foodsoft@f29105a into Foodsoft, so that settling an order can be hooked properly (after testing), then remove commit from our foodsoft repo with used by us.
• Test settling an order, and make sure transactions are made in the members system.
• Test that sending payment reminders still does the right thing.

[wvengen]

Note that the plugin moved to our foodsoft fork.