Warn users before they install any addons

In https://www.torproject.org/download/download-easy.html#warning, we warn people against installing additional addons or plugins. We should also throw up a similarly detailed warning when users navigate to the Addons pane, or try to install an addon from elsewhere.

In fact, I believe we already do throw up the generic warning, because we removed addons.mozilla.org as a trusted addon source. So perhaps we just need to change the text.