Allow the summary field to be optionally displayed

[msayoung]

Users reported that sometimes they dont want the summary field to display on a Page.
They were happy to fill it in for metatag purposes (if this is how its used) but would like to be able to optionally turn it off.

[finnlewis]

@markconroy suggests maybe hook_preprocess_node to check value and unset the in some case.

This is just on the full content display.

So:

• add a checkbox next to the summary field "Hide summary on page view" (default unchecked)
• add some hook_preprocess_node or something to work the magic

@ekes do we need an automated test for this.