Remove click-through EULA on install buttons (bug 743829)

What has been tested by hand:
- addons detail page (desktop and mobile)
- addons discovery detail page
- search results

I've successfully ran both the modified test files, but couldn't have all the qunit test run: they stop all of a sudden after the 8th and there's two CSP report logs in the console, no more details, please let me know if there's anything I can do about it.

There's one place where click-through EULAs are still present: in the add-ons pane on Firefox (when clicking on an INSTALL button in the search results). No clue if it's possible to modify this without touching Firefox's own code.

Here's what Unfocused told me on irc about that:

```
22:49 < Unfocused> magopian: you want to get rid of the EULA popup that the addons manager shows? file a bug
22:50 < Unfocused> the addons manager goes have a separate popup for that, but if there's a click-through EULA in the discovery pane then that comes from AMO
22:53 < Unfocused> er, and when you file a bug, needinfo both Mossop and I... things like EULAs are always trickier when its in-product UI :\
```

So before (or after?) merging, we should/could create a bug for this, thoughts?

Here's a screenshot of the EULA link added:

![eula_link](https://f.cloud.github.com/assets/167767/1885444/950e98a6-79cd-11e3-807e-c7ccb8228512.png)