Add a Custom time period to Leaderboard

[sgdavis1]

We would like to add the ability to display the leaderboard for a custom time period instead of the constrained ones in the homepage GUI. The API already would support this, and it is even partially supported by the client side code. You can see this from the console with a command similar to this:
loadLeaderboard(new Date("2016-01-01").getTime())
This doesn't support a few things we would want:

• Custom end date. (Currently always defaults to today)
• The time period label in the lower right of the table isn't kept in sync with the data.

Idea for UX: Both for minimal and full versions of the table, a click on the time period indicator label in the lower right of the table could bring up a popup or overlay that contains the date picker functionality. You could select a start / end / both / or none and this should correctly assume defaults for empty values. The widgets should maintain a consistent state with the selection of the time period from the dropdown (minimal) or menu buttons as well.

Challenges:

• There are not any built in date-pickers for SemanticUI so we would need to incorporate this first or else use a temporary solution (textfield with expected yyyy/mm/dd format or something). There is an example of this on the statistics visualization implementation.
• The client side JS would need to be reworked a bit for a sane implementation of the consistent label requirement. This is currently controlled by the string sent when the selection changes, but really should be enforced in the loadLeaderboard() method and set according to the dates picked.