[release/3.0] uses Arbeitspaket instead of Ticket

https://github.com/opf/openproject/pull/1521
https://www.openproject.org/work_packages/4936
Merge pull request #1525 from marutosi/finn/Arbeitspaket

[release/3.0] uses Arbeitspaket instead of Ticket
[release/3.0] fixes spec flicker by waiting for content (has_content?)