Allow events across multiple years

[bsoyka]

See python-discord/branding#171.

Currently, at least according to the validation in events/validation.py in the branding repo, events can't go from December to January as the end date is earlier than the start date (meaning this only affects New Year's).

This may or may not need some changes to the bot as well, but it's definitely worth some time and consideration as we still have a full year left to implement this.