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.