fixes asset path on timelines css
sets relative_url_root with instance variable value
fixes calling the work package description diff when in a sub uri
moves relative url root functionality into own module
uses the standard OpenProject way to set the prefix
streamlines configuration of relative_url_root

* includes relative_url_root into OpenProject::Configuration
* ensures loading in environment so that it is also used for asset precompiling
removes no longer necessary rake tasks
updates changelog