Better checking of namespace

We're aiming to get this into release-238 for release tomorrow. This PR is to dev, which will then be cherry-picked into the release branch.

https://wikia-inc.atlassian.net/browse/VE-1397
This would work for any usage of contenteditable - but isn't it meant to solve particular problem that happens with usage of VisualEditor? Using contenteditable is sort of implementation detail of VisualEditor and outside code should depend on it as little as possible - that's why we expose APIs and classes like the one I mentioned.