Discussions and posts handling

[matdon90]

Following functionalities with tests should be implemented:

1. Logged user can create discussion.
2. Discussion creator can edit discussion.
3. Administrator can close or delete discussion.
4. Logged user can add posts to discussion.
5. Post's creator can delete or edit post.
6. Logged user can get list of discussions.
7. Logged user can get list of posts of chosen discussion.