add additional 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.
Revert "add role "checkbox" to watch buttons"

This reverts commit 611445d81768d5b49e129d34c19a75e94234452d.

We were not sure if this is an improvement because checkboxes typically
do not change their labels on state change. So we decided to leave this
for now.