Bug 1033656 - Add reviewboard to mach mercurial-setup; version checking; r=smacleod

We want to make it turnkey for people to use reviewboard. So, we add
reviewboard and related functionality to |mach mercurial-setup|.

Since the reviewboard extension only works in Mercurial 3.0 and newer,
we add some version detection for the Mercurial version. This should
have been done months ago. We now have it.

I also took the opportunity to inform |mach bootstrap| that Mercurial
2.x is no longer modern.

I also updated the messaging around mq to encourage fewer new users to
use it. You may find this controversial. People can always ignore the
message.

Finally, I also added a histedit prompt to the mix, since a lot of
people don't know about that and many find it useful.

I could have broken this into multiple patches. Meh.

--HG--
extra : rebase_source : d33f8abcabb6ad6511c2f9e202283d43613fafc4
extra : amend_source : 3a56bc4d49ee6200cbdd0e87b4f28489518fee79