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.
If we ever run an "install an addon" snippet again, it better be after we've added about:home or snippets.mozilla.com to the install whitelist, so no.