Implement a new `/puzzle` slash command to create a puzzle

Implement a new `/puzzle` slash command to create a puzzle

This command doesn't (yet) accept any arguments. Instead, it pops up a
modal dialog to prompt for the puzzle name, ID, and URL.

The command also only works from a top-level hunt channel.

Some things that would be nice improvements here:

  * Auto-fill the puzzle ID based on the puzzle name without
    punctuation and spaces replaced with underscores.

  * Allow the command to work from the channel of any puzzle in the
    hunt

  * Fix the bug that the modal dialog reports an error even when
    things work correctly, (the problem is that the lambda is taking
    more than the maximum 3 seconds that Slack is willing to wait for
    a response).