Move setting of minor/watch defaults from MWSaveDialog to Target

It looks like it also came from there originally, because it uses
this.pageExists which doesn't even exist in MWSaveDialog. This caused
all pages, even existing pages, to be watched when 'watchcreations'
was set.

This logic really belongs server-side, though.

Bug: 56206
Change-Id: Idf500383b27a93136dc0cfdd60a2e7b2607af95c
Merge "Move setting of minor/watch defaults from MWSaveDialog to Target"