API: Add prop modules for backlinks

With the success of prop=redirects, let's create prop=linkshere,
prop=transcludedin, and prop=fileusage.

Let's also add a miser-mode-sensitive 'namespace' parameter to
prop=redirects, since the other three have a 'namespace' parameter using
the new *_from_namespace fields.

Change-Id: Ib650db9988a553771582eed0c183d0cae4b71252