(bug 45774) Add title.fileExists, improve title.exists

People sometimes want to know if the actual file exists, not just the
file description page. Support that.

Also alias .exists to .fileExists for the Media namespace, since that's
what the Media namespace is for.

And while we're at it, fix .exists for special pages too.

Bug: 45774
Change-Id: I019adc89858a1d32995a38d5e8eef32577fd32d6
Merge "(bug 45774) Add title.fileExists, improve title.exists"