HTMLForm: Handle Flags in non-OOUI form buttons, too

HTMLForm: Handle Flags in non-OOUI form buttons, too

Flags added to a button or submit field in a non-OOUI form will be
handled now, too.

With activated wgUseMediaWikiUIEverywhere:
All flags will get the 'mw-ui-' prefix.
(mw-ui-button is added to the button automatically)

With deactivated wgUseMediaWikiUIEverywhere:
All flags will get the 'mw-htmlform-' prefix.

This allows HTMLForm users to add flags to only the button, without
setting them as a cssclas, which would be added to the wrapper elements,
too.

Bug: T102838
Change-Id: I121a966fb4db6649a6e1012187148d354b5534f0