Add EditPage::getCopyrightWarning before "Send" button

Unfortunately EditPage::getCopyrightWarning returns wikitext, and
HTMLForm wants raw HTML, so we have to implement a simplified
version of OutputPage::addWikiText for our needs.

Bug: 54982
Change-Id: I0161e93e4f4f432878ab0b3e9aaca1a655f25c7d
Merge "Add EditPage::getCopyrightWarning before "Send" button"