Issue 2129: Most talkative day (end of year)

This commit has the following:

2 new methods in PostDAO:

1. getMostTalkativeDays: Returns the most talkative days in a range of days.
2. getAllPostsByUsernameOn: Returns all posts from a given day. I found that making the adjustment we'd discussed to getAllPostsByUsernameOrderedBy caused other insights to fail, and it seemed like a roundabout way of getting all posts from a single day anyway, so I went ahead and made a new method for it. I'm not 100% about how we handle things like timezones, so I would definitely give this one a close look to make sure it's doing what it's supposed to.

This one was a bit more complex than the others I've tackled so far, so I would assume there's plenty of room for improvement. Just let me know!