Implement training tasks for new users

Each new contributor of the wiki will be asked to first learn and practice in a "training wiki". Several training tasks will be set up for them, resembling real tasks that are shown in the Organic Data Science wiki. An example of a training task will be "Train John to assign participants to tasks". New users will practice in the wiki and will be able to edit tasks without disturbing the work in the Organic Data Science wiki.

@Felix_Michel: I believe you said this will be done by end of August, is that still the target? -- -- Yolanda