17683 migrate assignee and responsible to ui.select

[``* `#17683 ` migrate details pane to ui.select``](http://community.openproject.org/work_packages/17683)
- [x] switch version dropdown to ui.select
- [x] check accessibility (later PR)
- [x] fix tests
- [x] check how the layout and classnames change for different themes (this might affect test selectors if the theme is changed, ATM it's the select2 one
- [x] look for a solution for options list being entrapped in the parent block (it gets cut off if its height it bigger than its container; select2 was appending the options to body and blocking everything else with an overlay a-la jquery ui popups)
- [x] look for a possibility to add an empty option without prepending a null object to the list of options in the directive (none are acessible)