Accessibility Starter Kit: Open Sentencing

### Background on the problem the feature will solve/improved user experience
Open Sentencing needs to improve its web accessibility to be more inclusive of our end-users.

### Describe the solution you'd like
To this end in this project we'd like to:

- [ ] perform automatic testing to assess Level 1 accessibility (see reference issue)
- [ ] document and fix issues raised by automatic accessibility testing
- [ ] address any existing accessibility Github issues in the various Open Sentencing repos
- [ ] perform additional manual tests and fix those as well

### Reference issues
- #53

まず参照 issue #53 を読み、Open Sentencing リポジトリ全体のアクセシビリティ関連 issue を調査します。issue の説明から必要な自動チェックと手動チェックを特定します。完了とは、チェックを実施し、結果を文書化して修正し、既存のアクセシビリティ問題に対処することを意味します。

索引モデルが issue の本文から書いたものです。