Allow more than one weight entry per day

[rolandgeider]

At the moment, it's only possible to add one weight or measurement entry per day. While it doesn't make much sense to log this several times per day, it also doesn't hurt and it would make some of the logic more easy.

TODO: change the models from date to date time and remove the unique_together

[Dieterbe]

A trainer on our discord brought up a good use case: water cuts for making weight categories (eg in powerlifting comp)