Pages should be 'locked' when a user starts editing a page

If any other user (or the initial user, in a new tab) starts editing the page,
they should receive an alert that the page is being edited and to proceed with
caution.

![screen shot 2014-11-17 at 1 29 05 pm](https://cloud.githubusercontent.com/assets/111218/5078116/8cc51216-6e5e-11e4-9043-2773ce50dca0.png)

Fixes #364