bug 1549249 - hard-code new add-on signing intermediate so it's always available r=jcj,kmag a=ryanvm

Summary:
Our previous approach to making this intermediate available relied on being able
to add it to the user's NSS cert DB. This does work in the majority of cases,
but there are some situations where it doesn't work (e.g. if the user's DB is
set to read only, if they've configured Firefox to run in "nocertdb" mode, if
they have a master password but forgot it, and so on). This patch compiles the
intermediate in to Firefox in the same way we incorporate the root, so it should
always be available.

At the same time, this patch reverts the changes from
023dd959512e2cfa685187616560f91efa91183c and
1d35f8d88bdd007e01d42c4ff76c6d10d7c01a98 (the patches that implemented the
original approach) because they should no longer be necessary.

Reviewers: jcj!, kmag!

Tags: #secure-revision

Bug #: 1549249

Differential Revision: https://phabricator.services.mozilla.com/D30090

--HG--
extra : amend_source : dd475918be3f263a4a363c66a60edc708d3bdcca
extra : histedit_source : b6861a1d7c7ddbe07d5df73d76734d9a48ee3164%2C54cbc4b0446ff1ee3dc860bb2d3798ba8f662566