Signing Service initial support (bugs 1070189, 1070191, 1070227)

Bug 1070189: Call the trunion API (not tested with servers yet)
Bug 1070191: Signing management command
Bug 1070227: Script to sign all existing add-ons
Not sure having this helper (and the following one) is very useful. More indirection is ok when it simplifies/factorizes the code elsewhere, but it doesn't seem to be the case here: they're only used once, and only barely make the calls shorter in the `main` function.