Wrap extended post actions in ellipsis.

As discussed at https://meta.discourse.org/t/move-some-post-actions-to-extra-drawer/16094/18:

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Add a new SiteSetting to specify a maximum of items to be shown in post action menus per default. If more buttons are rendered and those after mentioned maximum will be hidden behind a collapsible ellipsis-button. Once clicked it slides in the missing buttons and hides itself.

If the setting is set to 0, the ellipsis will not be applied. It default is set to 4 though.

All buttons are created equal – but the Reply-Button is more equal than others: If it is rendered, the reply button will never be hidden behind the ellipsis. The max count is exclusiding the reply button and its position would make the reply button hide, it is removed there and pushed to the end of the list.