Bug 14970: Don't block our unsigned extensions

Mozilla introduced extension signing as a way to make it harder for an
attacker to get a malicious add-on running in a user's browser. See:
https://blog.mozilla.org/addons/2015/02/10/extension-signing-safer-experience
and https://blog.mozilla.org/addons/2016/01/22/add-on-signing-update/
for some background information.

In ESR45 this feature is enabled by default and we exempt both our own
extensions and EFF's HTTPS-Everywhere from this requirement.