mw.ViewPageTarget: Show AbuseFilter warning in save dialog

Misc:
* Updated signature of #showMessage to include allowing
  an array of nodes to be passed. We pass this to jQuery#append
  so we're just extending the explicitly documented subset of
  things it could already do due to passing to jQuery#append.

Bug: 50472
Change-Id: I3f56820a4f14b0684bfa265e3eb5e3820f2a3513