support Summary Card by Twitter

To know about Summary Card, read this document.
https://dev.twitter.com/docs/cards/types/summary-card

This change will make our event page Summary Card ready.
I did not test with validator, because it cannot parse webpage
which hosted on localhost.

Note that I hard-coded '@partakain' in this template because we have no configuration for administrator account. I know that `PartakeConfiguration` provides `adminScreenNames` but this can contain multiple value. I think this is not critical problem.