Open/close actions on admin/event

This adds the following actions to the admin/event page

* Open; For a selected event e, this opens all events x where x > e or x = e
* Close; For a selected event e, this closes all events x where e > x or x = e

where a > b indicates that a is a parent or a grandparent of b.
Merge pull request #102 from Prillan/reopen-action

Open/close actions on admin/event