Log voting timestamps about evap

[richardebeling]

We would only need (evaluation, timestamp) pairs to be logged. However, from the evaluation_evaluation_voters table and its transaction IDs, people with database access can currently deduce the order in which users voted. This would basically allow to correlate the timestamps with the insertion order, so effectively we'd also store who voted at that time. We probably don't want that stored? In this case, we need to do some additional work to get rid of the transaction IDs in the voters table, just as we did in #1549.

An additional table with an evaluation and a timestamp field, combined with insertion-order-obfuscation on evaluation_evaluation_voters, should be fine from a data economy point of view IMO.