properly implement /artist functionality

[softbobo]

The bot should be able to provide more info about artists playing at our events. We already have a blueprint for that function in our codebase, but we need to properly implement it:

• /next should return the artist names as hyperlinks. upon clicking, the artist() message is triggered for respective artist name and returns its DB entry (see better event-templates #31)
• nice-to-have would be, if one could trigger /artist with an artist name to return the info as well