Improve voice settings UX

Problem/Motivation

Currently the voice preferences at /admin/config/services/aws/polly are large and unwieldy.

Proposed resolution

• Use smaller buttons.
• Group voices into per-language details elements (so all the different Englishes will be within one details element).
• Use one demo text area per language (rather than per language-country pair).

Remaining tasks

User interface changes

API changes

Data model changes