Consider using prettytime for timestamps in forum search results

See https://github.com/scijava/scijava-search/pull/3#issuecomment-355495479.

We could make use of [prettytime](https://www.ocpsoft.org/prettytime/):

```
<dependency>
   <groupId>org.ocpsoft.prettytime</groupId>
   <artifactId>prettytime</artifactId>
   <version>4.0.1.Final</version>
</dependency>
```

