Bug 862311 - [MMS][User Story] Display attachment icon in thread

Additionally, define the thread list item HTML via a template string.
Add mock data for demoing in desktop environments
Simplify thread markup

Since the `unread` status of a thread describes the thread as a whole,
it should be declared on the top-level DOM element. In addition to
allowing for more flexibility with styling, this approach simplifies the
thread's template string.
Add performance optimization
Correct template usage
Tweak styling
Incorporate Corey's feedback