Add property flag_message_html to store if the flag/unflag messages should...

kensaerequested to merge
issue/flag-3563820:3563820-allow-html-in into 5.x

Add an extra form checkbox to choose if html code should be allowed in the flag/unflag messages. This setting will be stored in a new property 'flag_message_html' on the flag entity. We capture this setting in the ActionLinkFlashCommand to modify the message text behavior in js.

Related to #3563820

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