category proposal events

The category events view introduced in #883 is missing some events because it is not trivial to collect all events related to a category. This pull request improves the situation by adding support for proposal events and one level of subpage events.

Some context:

-  Each event has `topic` (`model.Delegateable`).
-  The interesting thing about proposals is that they consist of two delegateables: The proposal itself (`model.Proposal`) and a `description` (`model.Page`). The category badge is related to the proposal itself while comments are related to the description. So with the previous query proposal comments were not included.