[SMS App] Stop ChatMessage.qml from rendering HTML
Summary
Make ChatMessage.qml treat incoming text as Plaintext
In the future, I will return to having some kind of formatting (so that we can do link highlighting, at least), but for now this will resolve a variety of strange issues.
Test Plan
Before:
Various things were going slightly wrong, like newlines being ignored and < characters causing either complete message annihilation or strange formatting.
After:
HTML elements are no longer considered, preventing messages from being unexpectedly mangled.
