Add a single delete command for all data

[cascer1]

To improve user privacy, anyone should be able to easily remove all their stored data.

Data that can be deleted:

• Nightscout settings (URL, token)
• Quoted messages (i.e: messages attributed to the user)

Possible commands:

• /delete all delete all data associated to the user
• /delete nightscout [all/token/url] delete specified Nightscout data
• /delete nightscout privacy set Nightscout to private in all guilds
• /delete quotes <this guild only(boolean)> delete all quotes attributed to the user. If boolean is true, only delete in current guild