Bugzilla on GitHub and Dual-Life Extensions

(tl;dr: Bugzilla git primary is going to be github. git.mozilla.org for
bugzilla development is going away; Useful extensions are getting their own
repos.)

For a variety of reasons, the primary git repository for Bugzilla will be
moving to github.

Our code is already on github (mirrored) but in the coming weeks we'll need
to update our remotes to push to it as well.

As it would happen, a benefit arises from this:

Currently there are several very popular extensions whose canonical home is
... the bmo repo.
This is not desirable for a number of reasons:

1) If someone contributes an improvement (rather than a fix), that
improvement has to be vetted both on the principal "is this what is best
for the extension?" and *also* "Will this break bmo?"
As a result, improvements to extensions linger with very long review times
(this is but one reason reviews take a long time)

2) It points people at the bmo repo. The BMO repo is not consumable. It's
possibly (and encouraged, I think) to take ideas/code from it, but anyone
trying to run it that isn't Mozilla is burdening themselves with a lot of
arbitrary decisions and it will almost always end in tears.

3) Sometimes the BMO team breaks compatibility in an extension, as has
happened with EditComments (as I recall).

4) The commit history of the extensions is littered with the rest of the
commit history in BMO

Thus, since we are moving to primary git hosting to github anyway, I am
going to split out (as a start)
InlineHistory, EditComments, and SecureMail as git repositories under
https://github.com/bugzilla.
These will be history-complete with the ones on BMO, but because of the
magic of git-filter-branch the history will be useful.

Astute observers will note that extensively used extensions should be
core'd -- included in the regular bugzilla codebase. Those observers are
correct, but doing so is often a non-trivial amount of work.