OOUIHTMLForm: Implement HTMLMultiSelectField

Authored by matmarex.


OOUIHTMLForm: Implement HTMLMultiSelectField

Example usage in I30d401cc7c827b21eb2fb116558d0d9e764ec1f3.
Following the template set by HTMLCheckMatrix, which for some reason
was implemented first. Also removed unfulfillable @todo comment.

Also corrected HTMLCheckMatrix code not to wrap OOUI widgets in
MediaWiki UI wrappers even when 'UseMediaWikiUIEverywhere' is set to
true, and to set the correct 'value' for checkboxes.

Bug: T100955
Change-Id: Ib5d000ca9a08abc8086ee05b5122116b086242ad