mediawiki.htmlform: Preserve classes added by HTMLForm's cssclass option

Follow-up to commit 8a9341b. When changing the multi-select
checkbox container to a select multiple, preserve any custom
classes added via the cssclass argument for the htmlform field.

Change-Id: I41d2a306627b33b34d7e464147d771cc727f4f61
Merge "mediawiki.htmlform: Preserve classes added by HTMLForm's cssclass option"