Tighten restrictions for "exiled" users: allow to see only their own content

[Aivean]

Now when user loses all permissions, their access to the content is limited only by their own posts:

• Can still see the list of subsites
• All feeds (including feeds by subsite) show only user's posts (every feed is the same)
• Feed sorting still works
• "Fire" works, but is limited to user's own posts
• Cannot see other people's profiles
• Doesn't have access to other people's posts, even by direct links

[Aivean]

@4vanger, addressed (some) review comments, please take a look.