Allow opt-in for storing data

[ytturi]

Following #39 ,
The bot will collect user data (username and group_id), when the database is used.
This should be easily accesible by any user using the bot (i.e. /help) and have an opt-in capability instead of working by default when the database is set on bot-level.

The way to do this would be to have an "allowed groups" table in the database and an /opt-in command (or similar).
When the bot enters the group, send a message informing of its usage.

• Allowed groups table
• opt-in command
• opt-out command (and/or detect when the bot leaves the group)
• markdown file with data usage expalanation
• help command explaining general usage as well as data usage link