Correct button colours of preference page when $wgUseMediaWikiUIEverywhere is enabled

* Apply mw-ui-destructive to Special:Preferences/Reset
  when $wgUseMediaWikiUIEverywhere is enabled

Introduces HTMLForm->setSubmitDestructive()

Bug: 65317
Change-Id: I1d6691dce3e7dab662bda9a718e16c5caee6c041
Merge "Correct button colours of preference page when $wgUseMediaWikiUIEverywhere is enabled"