Allow trigger items on admin task (sequence)

[GDay]

This will allow admins to add a new block triggered by "admin tasks". Previously conditions could only be set up based on time (before and after the new hire started) and on to do items the new hire had to complete. Now we can also trigger items based on an admin task.

Please note that you can only select admin tasks that have been created in the same sequence. Admin tasks do not have a "template" and cannot be reused between multiple sequences.

Todo:

• [x] Add tests
• [x] fix duplicating sequence (wrong trigger on admin tasks)

[coveralls]

coverage: 93.316% (-0.06%) from 93.378% when pulling 6cef66109e7326a81c1a5133abbed0dcb39ee1d1 on allow-trigger-on-admin-task into 80bf55996e1dce08f36fbc39234b197e1e4fb096 on master.