add users editable support

[``* `#17014` users editable``](http://community.openproject.org/work_packages/17014)

# TODO:
- [x] change placeholder to minus
- [x] move overview.html conditions to a proxy/decorator builder/dispatcher (Separate PR)
- [x] use name instead of title
- [x] autoopen the select2
- [x] check select2 grouping docs (probably select2 will be replaced)
- [x] rebase
- [x] style as inline (implemented in another PR)
- [x] fix protractor tests