Implement transfer screen

[jlherren]

Before this can be done, a thorough list of use-cases needs to be established, since multiple types of transfers are required, including at least:

• [x] Transfer reflecting a real-world money transfer between two users (e.g. pay up for past lunches with real money)
• [x] Transfer of points or money from one person to another, due to some agreement (e.g. a bet)
• [x] Transfer of points and money going in opposite directions between two users (e.g. buy/sell points for money)
• [ ] Transfer of points (or money?) from one person to everybody else (e.g. penalties). This begs the question, who is "everybody else"?
• [ ] As per #7, transfers can be used to settle direct or additional lunch payments

One option is to have a generic form that allows all use-cases at the same time, but that is likely to be prone to errors by the user. The saner approach is probably to have a selection menu where the user can select a concrete use-case and then a form tailored to that use-case would appear.

[jlherren]

About the "fine" scenario: Since defining who "everybody" is might be difficult, one option is to not define it at all and to list all recipients manually. This allows to also create fines from which only directly affected people will profit. On the wizard screen there could still be a button "Add all users", but in that case checking the user's active flag is good enough.