[bug 904443] Shorten contributor forum excerpts

The post_content is an array of strings--one for each post in the
thread. This changes excerpting so that we only use the first
post in the search results summary.

To test:

1. do an advancd search in the contributor forums for "plugins" and notice summaries are really long and consist of all the posts in that thread
2. apply the patch
3. redo the search and notice summaries consist of just the first post in the thread

r?