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.~~ Thanks, @nkenneweg.