Fix/timelines api migration

Wraps up https://github.com/opf/openproject/pull/527:

> roll-up branch for migrating the rest of the API-views to RABL (given, that we actually make things faster...)

This one also restores some timelines functionality. Unfortunately, Timelines are still missing the capability to fetch users trough their new API, which is why this PR explicitly leaves [`#2577`](https://www.openproject.org/issues/2577) unfixed.