user set known: DM prompt to give opportunity to set

[synrg]

Dronefly has the privacy-protecting measure that unless explicitly opted in by the user by setting [p]user set known True, the bot will not carry their settings over to other servers the user may be in where Dronefly is present.

This issue is about providing the user with a DM prompt from the bot offering them the opportunity to turn on the feature. The prompt will be triggered the first time the user gives the bot a command that acceses their user data on a server where they are known, and they don't have the setting turned on.

In the message, they should be offered to turn on the setting or keep it turned off, e.g.

Would you like Dronefly to know your settings on other servers you join where it is present? Answer yes to turn this on. Answer no to leave it turned off (the default) and you won't be prompted again on this server. Answer never if you don't want to be prompted about this setting again, either here or in any other server.

[synrg]

After the prompt is answered by the user, the bot should answer with a confirmation message that their response was acted on. Also finish with this note appended to the confirmation:

If you want to later change this setting, use [p]user set known False to turn it off (if they answered yes) or ... use [p]user set known True to turn it on (if they answered no or never).

[synrg]

This helps with self-serve & usability for the first public release. Adding to project.