Make imagelinks work like templatelinks

Due to bug 17259 (which is fixed in a better way in Id44d566a),
imagelinks only records the redirect and not the actual image used when
an image redirect is referenced. This causes various problems, such as
cascading protection not working through image redirects.

It makes more sense for imagelinks to work like tempaltelinks, recording
both so that things like cascading protection don't have to care about
image redirects explicitly.

Comparing imagelinks to templatelinks also reveals a few places
(WikiPage::doDeleteUpdates, WikiPage::doCascadeProtectionUpdates) that
should be triggering a LinksUpdate if the image links changed.

Bug: 23002
Bug: 23542
Bug: 26503
Change-Id: I64fe7d25646cae2c8213211893c6f821f3504dbf