[Feature Request] Enable reminders to be set within DMs

[jeremyle56]

Currently if you try to set a reminder in the DM with the bot, it will produce an error as it classifies the DM as a private channel.

Private reminders would be a valuable feature. You can set the reminder within the DM with the bot and then it will remind you with a DM message.

[francojreyes]

Notes for me:

• Get rid of the global DMs disabling thingy
• Disable certain /settings subcommands in DMs using @guild_only
• Make generation of /settings view and /help settings depend on channel type
• Set guild_id to None for reminders in DMs
• Handle execution of reminders differently based on whether guild_id exists
• Make fetching of reminder list for /list and /delete return DM reminders