Tag Management: Delete Flow

Closes #4633
- The ‘delete’ button is now a smaller plain text link, opening a
confirmation modal analog to the delete user/post flow
- Adding a post count is dependent on #4654, but the modal is already a
neat step up from the immediate, warning-less deletion.

![screen shot 2014-12-15 at 9 07 08 am](https://cloud.githubusercontent.com/assets/1426799/5440036/dd65c6de-8439-11e4-83cc-80e87130cdc4.png)
![screen shot 2014-12-15 at 9 07 15 am](https://cloud.githubusercontent.com/assets/1426799/5440037/ddde4334-8439-11e4-94f1-3abc3ac74dd0.png)