Add `/subreddits` command

[TimJentzsch]

Add a /subreddits command to show the most often transcribed subreddits of a user.

The response of the bot could look like this (taken from tor-user-stats):

The graph shows the number of transcriptions completed on the subreddits. It should only show the 5-10 highest entries, the others should be summed up to "other".

Unfortunately we don't save the subreddit directly, it can only be extracted from the URL of the post. So we probably can't run a query on all submissions but have to analyse each one individually. Therefore, we have to restrict ourselves to the last 500 posts to save API calls.

Options:

• username (optional): The user to get the subreddit graph for. Defaults to the user executing the command.

[TimJentzsch]

Maybe this should be names /sources instead to stay more independent from Reddit.