(bug 66960) Require MediaWiki:Licenses to enable uploads (WMF)

This change checks whether MediaWiki:Licenses has any text.
By default it does not, which means by default uploading is disabled.
To enable, the page needs to be simply created.

The advantage is that some or many small WMF wikis still do not have
license options, which is really needed for our copyright requirements.
https://wikimediafoundation.org/wiki/Resolution:Licensing_policy

A previous attempt in core was unwanted, see I465daaff4aed4b484a1f4707fe57d45c0e9ce6cd

Change-Id: I364f48f387d5987ba2b93e90ea6b2a3c5de1461d