Group topics into categories

@peterkos's suggestion:
> [It] might be cool to have separate categories of topics: a set for langs, a set for frameworks, a set for design help, a set for "how to build X" (where to start building backend, app, etc.)

This could be implemented by adding another layer to the tree structure:
```
Server
 |_ Category
    |_ Topic
        |_ LinkedList<Member> queue
```