Bug 1063170: Add static HTML page for addon-installing snippet

In [bug 1063124](https://bugzilla.mozilla.org/show_bug.cgi?id=1063124) we're working on putting up an about:home snippet that can directly trigger an add-on install. While [bug 1046903](https://bugzilla.mozilla.org/show_bug.cgi?id=1046903) is for adding that ability to about:home directly, in the meantime the only way to do this is to have a static page on addons.mozilla.org that triggers the install.

This PR adds that static page for inclusion via an iframe. This particular snippet is for installing LastPass.

Also includes a small fix to the install.php script that I noticed while first attempting to use it before we settled on a static page.
Will update the URL format when I'm back from PTO.

> I wonder if these hardcoded URLs should be set in Django's settings. Or at least reusing the domain part?

Given that we can't guarantee LastPass will be on the other environments, I'd rather not.