Embed factory/pycord fixes

[classabbyamp]

• common.py: don't error when creating embeds for users without avatars
  • behaviour changed in pycord 2.0: https://docs.pycord.dev/en/stable/api/models.html#discord.User.display_avatar
• common.py: use tz-aware datetime for proper timestamp display
  • https://docs.pycord.dev/en/stable/api/data_classes.html#discord.Embed.timestamp

fixes #467