Fix the display of the leaderboard for user having no badges

For user with no badges, ie no rank, we need to retrieve the list of users
with badges and extend this list with the user without badges but then
ordered by the date of creation.
This allows user with no rank to still see themselves in the leaderboard.

Fixes: https://github.com/fedora-infra/tahrir/issues/135
Merge pull request #231 from fedora-infra/feature/leaderboard_ranking

Fix the display of the leaderboard for user having no badges