Fix "Medium severity vulnerability found in bootstrap"

We currently have a vulnerability (as reported by snyk) because of Bootstrap 3.3. The proposed fix is to migrate to Bootstrap 4. We could also wait for a release of 3.4, where there is a fix, but I'm not certain when that comes (if ever).

We only use the CSS of Bootstrap, so this is probably a non-issue. But it is annoying to have Snyk report that it is (and it doesn't look good that it does), so we might want to do it anyway.

まず Bootstrap 3.3 の依存関係と issue に記載された CSS の使用箇所を特定し、リンク先の Snyk の脆弱性を確認します。プロジェクトを Bootstrap 4 に移行できるか、または Bootstrap 3.4 を待つべきかを確認してください。選択した依存関係によって報告された脆弱性が発生しなくなれば完了です。