additonal category views

This adds three new views on categories:

-   truncated description (`/description`)
-   related events (`/events`)
-   related upcoming milestones (`/milestones`)

Those are mostly useful for embedding.

`/events` has a similar API to `/events/all` (`event_filter` and `count`). `/milestone` does also interpret `count`. You can also set `enable_sorts` and `enable_pages` in both URLs which may be useful for embedding.