support for leap day birthdays in non-leap years

[seeeeew]

Currently leap day birthdays in non-leap years work as expected in bday list and bday next, but the bot won't send out alerts for them, since the date is simply skipped. This behavior isn't wrong, but also not optimal.

[seeeeew]

The best solution is probably to ask the user if they want to get their birthday message on the 28th or the 1st. Just need to decide how to ask them and how to store the information.

[seeeeew]

• New column leap_adj (-1/0/+1, default 0) in table birthdays.
• New hidden command <prefix>bday leap <value>:
  • <prefix>bday leap before – send birthday message on 28 February in common years
  • <prefix>bday leap after – send birthday message on 1 March in common years
  • <prefix>bday leap reset – don't send birthday message in common years (default, at least for now)
• Command won't appear in the help, instead the bot will tell users about it when they set their birthday to 29 February.
• Bot will show the actual birthday in lists, but internally work with the adjusted birthday.