Make sure mediawiki.notification is loaded -before- calling ViewPageTarget#checkForWikitextWarning

That function needs to return the result immediately, not wait for module loading (via mw.notify).
This was breaking us being able to keep track of what wikitextWarning object was in use (but only
the first time we used the module), and therefore fail to close the warning when the wikitext
disappeared.

Bug: 70168
Change-Id: I0f1427423a5fe82ec8e70e2f0462a3044ca7ace8