[HTTPS-Everywhere] Firefox to require extensions be signed by addons.mozilla.org

[Jacob Hoffman-Andrews]

FYI, Mozilla just announced they are planning to require all extensions
be reviewed, approved, and signed by addons.mozilla.org before they can
be installed from a third party website:

https://blog.mozilla.org/addons/2015/02/10/extension-signing-safer-experience/

We have already submitted HTTPS Everywhere to AMO, and are going through
the (slow) review process, so this isn't a huge change for us. Hopefully
the process to get a signature for a third party website is much faster
than the process to upload a new version to AMO. Otherwise we will be
faced with a very slow release process.