Disable unavailable extensions (bnc#883148)

I see there two checks for false. It have impression that nil is in fact available. I found quite confusing to see that `available = nil` in fact is available. I recommend to hide such logic into addon class like removing this delegator and instead have
```
def available?
  # nil mean, that server do not report about availability, so we expect that all is available
  !(@pure_addon.available == false)
end
```

and then later use intuitive things like `unless available?`