Use FormSpecialPage for Special:RandomInCategory

This commit moves [[Special:RandomInCategory]] from using the SpecialPage class
to using the FormSpecialPage class, in the process moving from a simple input
box in the middle of a sentence to a "proper" form.

Bug: 67581
Bug: 67253
Change-Id: Ibf8694c0643f7cd3964ff9f62ffc6049f0978416
Merge "Use FormSpecialPage for Special:RandomInCategory"