Show accept terms of use message

This adds message asking the user to accept the "terms of use" before using the application.

The current text is a placeholder. @HowardLook, I can switch it out with whatever you think is appropriate.

Currently, we **only show the overlay after signup**. I thought it would be a little annoying to have it at every login, but if that is required I can easily toggle it on for login as well.

I've added a `SHOW_ACCEPT_TERMS` config variable to easily turn it on/off (could be useful for local development).

Also added an end-to-end test to make sure it pops up.

![screen shot 2014-04-07 at 5 26 26 pm](https://cloud.githubusercontent.com/assets/1306536/2633362/d7a0fca6-be6d-11e3-8273-e2e23c1d2163.png)


Side-note: @jh-bate you might be interested in taking a look at the overlay CSS (especially the use of `position: fixed` and `overflow-y: scroll`, for your messages modal.