(bug 42915) make MovePage aware of whether redirects are supported.

Special:Movepage and ApiMovePage should be aware whether the present content
model supports redirects, and only offer to keep a redirect on moving
of it's supported.

Change-Id: I2d02b2668ea8ebe39387b79d165f6cf1037303b9
Merge "(bug 42915) make MovePage aware of whether redirects are supported."