Merge release 3.0 into dev

This PR merges all changes made for packager.io on release/3.0 branch into dev.

Please be aware that there is an additional [PR 1857](https://github.com/opf/openproject/pull/1857) which includes this changes and additional to handle bower related stuff. The idea is to merge [PR 1857](https://github.com/opf/openproject/pull/1857) after this PR.
N.B. we're using the backported Rails 4 asset pipeline (see c0dc50e9c083481a6ff457bb6e7cde027038d1af) and no longer use the `assets` group in our Gemfile (1b8354972c2fd7c6bb67683184d7404ca24a0bda)