wall-network: refactor access permissions
This commit changes the access permissinos to the wall and network tabs, as well as posting to the wall and commenting on activities. Here is a summary of how things should work now:
- Wall: should be visible for every logged user. Only friends/members are able to post scraps.
- Network: only the user himself/herself can see the network.
- Activities: every logged user can comment on activities.
Closes #224 (closed)