[Twitter] fix 404 bug, convert to Data.Text

- If a user account became unavailable (deleted, etc) instead of just
  unfollowing, we receive a 404 when we go to look up the username.
  Rather than bombing out at that point, we now catch status code
  errors in the Twitter username fetch function, and return mempty for
  that user, which should cause it to just be removed from the cache.

- The Twitter datasource also uses Data.Text now instead of String.

Signed-off-by: Ricky Elrod <ricky@elrod.me>
Use Data.Text in the runner program a bit more

Signed-off-by: Ricky Elrod <ricky@elrod.me>