Add transferred/received amounts in statistics page

[zorun]

[zorun]

Names are temporary, see discussion in #1299

I have tried several orders for the column, this one seems clearer to me, but I'm intestered in feedback.

The negative signs are intended to show that all columns should sum to the balance.

[zorun]

Some ideas for improving the rendering after showing this to somebody:

• there should be two clearly separated sets of columns:
  • me and the group: how much did I spend for the group, how much did the group spent for me
    • one column with "payment/contribution" (currently called "Paid" in my screenshot above)
    • one column with "expenses involving me" (currently called "Expenses" in my screenshot above)
  • money transfers: individual transfers of money between participants to restore the balance of the group (partially or completely)
    • "transfers"
    • "received transfers"
• the two sets should be easy to understand, with good names and maybe an icon
• if it's clear enough, there might not be any need for negative signs anywhere

[almet]

Hi, thanks for the proposed changes. I'm not sure you need a review, or do you want to keep on adding things? 🤔

It could be improved, but at the same time it feels like it could just already add some values to the stats page.

[almet]

I've just rebased this on top of the latest master branch and updated the naming to follow our discussion in #1299

[almet]

(I plan to merge if/when tests are green)

[almet]

The tests should be passing now, but it seems that they change the way the balance is working.

Before these changes, a reimbursement to somebody would remove the amount from how much somebody has paid, and now it's not the case anymore.

I'm for merging this, as it seems to be the "good" behavior, but that might be a surprise for some clients.

Because it's changing the API we're exposing, we probably should issue a major release with the next release.

As this is changing some code you did @zorun, it would be great if you can review the changes I did.
Thanks!