Improve slippage investigation during weekly payouts

[fleupold]

As part of the weekly payouts, reviewers check a couple of transactions for correct slippage accounting. In order to facilitate this process, it would be good to

• Have a query that allows to see total slippage (in $) per solver and token (to find candidates for manual price correction)
• After merging Add multiple price feeds option #65, we should have multiple price feeds that determine the computed slippage value. It would be good to display all available prices (with their source) in the raw slippage query (together with the final price that was used for computing the displayed value)