Allow users to add unverified extensions if they acknowledge the risks

[TsukiZero]

Because some developers for extensions prefers to release versions on GitHub and the like before pushing it to Firefox's Add-On and Extension repositories, and there is no way to do so other than temporary addition (which is not a good solution), using Nightly build (also no good), or Developer build (EVEN WORSE).

And really, some of us enabled debug mode because we know a part of the risks involved already, so allow us to walk the rest of the way if we want.

[Jon]

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