info link on metrics

Metric selection panels should have info link (if available) which provides a summary of the metric in a modal window.

To support this, the calculated metric creation, and events definition should offer summary field (which is the text shown in the modal)

This would help

* non-regular users easily view/understand definition of metrics such as exits and exit link instances

* allow admins to be descriptive of event/calculated metrics where metcis name not clear or could be mis-intrupreted.