Bahá'í date functions

[dragfyre]

Add the capability to convert to/from Bahá'í dates. Should include the following:

• /date command that auto-detects a date in Badi' or Gregorian format and converts to the opposite format
• Toggle to display the Bahá'í date at the top of the server's channel list (off by default except on Bahá'í.FYI / Baha'i CoLab), similar to the following screenshot:

And of course, there's bahai-date-api that can be used to provide this functionality. (a docker build is now available thanks to @mdonaberger: https://hub.docker.com/r/unmolded1457/bahai-date-api)