Add letter-sound learning events

[jo-elimu]

• [x] https://github.com/elimu-ai/model/issues/273
• [x] https://github.com/elimu-ai/webapp/issues/1624
• [x] https://github.com/elimu-ai/analytics/issues/127
• [ ] https://github.com/elimu-ai/content-provider
• [ ] https://github.com/elimu-ai/herufi
• [ ] https://github.com/elimu-ai/kukariri