Ask for the user's consent before creating them a Slack account

I'm referring specifically to codeforphilly.org's install; I'm not sure how much the running version diverts from this codebase.

Basically, Slack has a [privacy policy](https://slack.com/privacy-policy) and [terms of service](https://slack.com/terms-of-service/user) that users implicitly agree to when they create a Slack account. codeforphilly.org creates the user a Slack account upon their registration for the site without warning or asking them.

I'm not actually sure if it happens upon registration or upon browsing to `https://codeforphilly.org/chat` while logged in. For the former, I suggest adding an opt-in or opt-out tickbox for Slack to the registration page which links to Slack's terms of service and privacy policy. For the latter, I suggest placing a intermediary page between `/chat` and the redirect which asks for the user's authorization, again linking to the Slack terms of service and privacy policy.

まず、codeforphilly.org のインストールで、登録からログイン済みユーザーの /chat へのリダイレクトに至る Slack アカウント作成の流れを追跡し、その後、それらの経路をこのリポジトリと比較します。完了条件は、ユーザーに警告が表示され、Slack アカウントの作成を承認または拒否でき、Slack の利用規約とプライバシーポリシーへのリンクがあることです。

索引モデルが issue の本文から書いたものです。