Display a generic description for topics in search

We'd like to return topics (which used to be called specialist_sectors
and still are in the format name) in search.  However, these have no
metadata or description, so they look quite confusing in search.  To
make them actually look like a search result, this commit adds a generic
description for each topic result which is missing a description.

Before:

![image](https://cloud.githubusercontent.com/assets/73564/5412444/907821be-8204-11e4-86c3-516fe07b5c47.png)

After:

![image](https://cloud.githubusercontent.com/assets/73564/5412473/c0d17662-8204-11e4-8d12-a8ae902040b5.png)