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`.

Below is an example usage
![before](https://f.cloud.github.com/assets/1675190/1641140/2145ca3c-584b-11e3-8dd4-5ff752d1f016.png)
![after1](https://f.cloud.github.com/assets/1675190/1641141/2411e58e-584b-11e3-93b6-daf83bb43967.png)
![before2](https://f.cloud.github.com/assets/1675190/1641142/26224bd4-584b-11e3-9e2c-a1a56aec423d.png)
![after2](https://f.cloud.github.com/assets/1675190/1641143/27e1f7ee-584b-11e3-8886-64c4243d8348.png)