On the `/filter` route, the categories filtering query language is now
supported in the input per the example provided below:
```
category:bug => topics in the bug category AND all subcategories
=category:bug => topics in the bug category excluding subcategories
category:bug,feature => allow for categories either in bug or feature
=category:bug,feature => allow for exact categories match excluding sub cats
categories: => alias for category
```
Currently composing multiple category filters is not supported as we
have yet to determine what behaviour it should result in. For example,
`category:bug category:feature` would now return topics that are in both
the `bug` and `feature` category but it is not possible for a topic to
belong to two categories.
0162f0ccb0