Change teasers on threads to preview the first post made

[jwjulie]

Change src/topics/teaser.js, which handles parsing of topic posts and replies to make a thread teaser, to show the first post in a topic to appear as the teaser.

Effort: Low, a few days
Dependencies: None
Acceptance criteria: As per title. The first post in a thread is either the question asked by a student poster or announcement made by staff. Currently threads show the most recent reply.