Add challenge feature for initiatives

[IngmarM]

As the owner of an innovation initiative, I would like to be able to create a "challenge" for my colleagues, in order to get the best solution for a topic that is relevant for my initiative.

Challenge attributes:

1. Challenge name & description
2. Challenge owner
3. Due date (optional)
4. Reward for best solution (optional)
5. Solutions
   • Solution description
   • user
   • upvotes / likes
   • comments (for the solution)
   • optional: Mark as winning solution

• Challenges shall be part of the collaboration tab on the initiative page.
• Challenges shall be shown in the news stream.
• New solutions to the challenge shall also be shown in the news stream.