API: Add prop=redirects and list=allredirects

While redirects can be sort-of queried using list=backlinks with
blfilterredir=redirects, we can get more accurate results with a module
dedicated to this purpose. We can also get the fragment of the redirect
without having to load the content of the redirect page and parse it.

I'm a bit surprised I was able to put together a query for this that
will work as a prop module. Or did I overlook something?

And then we may as well add the corresponding list=allredirects, to work
like alllinks, allfileusages, and alltransclusions.

Bug: 57057
Change-Id: I81082aa9e4e3a3b2c66cc4f9970a97eed83a6a4f
Merge "API: Add prop=redirects and list=allredirects"