Fix status code (source-academy#957)

* fix status code

* update deps

* change credo setting

Bump Elixir and Erlang versions (source-academy#946)

* Reformat README

* Update GitHub workflow Elixir and OTP versions

* Update README versions

Bump Elixir and Erlang versions (source-academy#946)

* Reformat README

* Update GitHub workflow Elixir and OTP versions

* Update README versions

Contest leaderboard hiding (source-academy#826)

* Add contest leaderboard hiding

* fix open leaderboard wrong condition

Contest leaderboard hiding (source-academy#826)

* Add contest leaderboard hiding

* fix open leaderboard wrong condition