Groom database to remove stale memes

[leviable]

The current implementation of Dankbot stores all memes in a database that have been successfully posted to Slack. When a new Dankbot run is started, and memes are pulled from subreddits, their links are first compared against the database, and any memes already in the database are removed. While this approach works, it allows the database to grow in an unbounded manner, as stale memes are never removed from the database.

Alter Dankbot do do the following:

• Add a "last seen" column to the 'memes' table, which will contain a datetime string
• If a meme is found in the database, update it's datetime string
• Add an entry to the dankbot.ini to specify how long, in days, a meme entry must be stale before it is groomed out of the database
• Add a new method to Dankbot that grooms the database, removing any memes that haven't been seen in X days, where X is the number of days specified in the dankbot.ini