[81507580] minor formatting change to tariff updates' date

Add `tariff` formatting for `Time` objects. Uses `dd/mm/yyyy` allowing to copy & paste inside rollback *datepicker*.
Also add some parsing of times for `TariffUpdate` and `Rollback` models to keep all times in the same format.

[PT](https://www.pivotaltracker.com/story/show/81507580)