[Feature] Add "participants" to each expense

[BloodRaven0]

Example: people A, B and C are on a holiday together and have a group in the app with all 3 people in it. Adding items to the expenses splits them evenly between the 3 of them. However, if people A and B decide to go out drinking and C prefers to stay in the hotel, adding the bar tab in the app would have C pay for something he took no part in. In that case A and B would have to make a separate group that would only complicate things further. And it gets even more complicated if there are more people involved.

So I propose a simple option when adding an expense: Participants, which lists all members of the group with a tickbox next to their names (by default all will be checked).