[#140] Rename "subscriber responses" -> "hotline messages"
Motivation
We have gone through many iterations of what to call messages from subscribers. We have finally settled on hotline messages and this PR does the work of standardizing the use of that throughout the app!
Main changes
- Migration/model change
- Variable naming throughout app
- Semantic message changes in each language (EN/ES/FR)
Closes #140 (closed)