introduce weekdays to fixed_hours tariff

[tpd-opitz]

https://forum.openwb.de/viewtopic.php?t=11934

• extend config fail save
• update overlap detection
• update current price fetch

TODO: update UI

• add list of weekdays a time frame should be active
• values in this list range from 0 (monday) to 6 (sunday) eg:

              [
                  {
                      "name": "tariff1",
                      "price": 100,
                      "active_times": {
                          "dates": [("01-01", "31-12")],
                          "times": [("08:00", "12:00")],
                          "weekdays": [0, 1, 2, 3, 4],  # active on weekdays
                      },
                  },
                  {
                      "name": "tariff1",
                      "price": 200,
                      "active_times": {
                          "dates": [("01-01", "31-12")],
                          "times": [("08:00", "12:00")],
                          "weekdays": [5, 6],  # active on weekends
                      },
                  },
              ]