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

Comienza leyendo el issue de referencia #53 e inspeccionando los issues relacionados con la accesibilidad en los repositorios de Open Sentencing. Determina las comprobaciones automáticas y manuales necesarias a partir de la descripción del issue; hecho significa que las comprobaciones se realizan, los hallazgos se documentan y corrigen, y se abordan los problemas de accesibilidad existentes.

Escrito por el modelo de indexación a partir del texto del issue.