BAC-551 Hide category namespace on Special:MovePage

**PLEASE DON'T MERGE WITHOUT APPROVAL FROM** @Wikia/platform-team

This is a backport of 3 commits from the latest MediaWiki release.

I used `git format-patch` and `git am --signoff` to apply the commits.

See [BAC-551](https://wikia-inc.atlassian.net/browse/BAC-551) for details.