Alerts can now have buttons, that go to some url when clicked. Yay.

Implementing that was a PITA, because Yesod really only wants its type-safe
urls to be rendered from within its Handler monad. Which most things that
create alerts are not. I managed to work around Yesod's insistence on this
only by using a MVar to store the pure function that Yesod uses internally.
That function can only be obtained once the webapp is running.

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Fixed a nasty bug where using gpg would cause hangs. I introduced this back
when I was reworking all the code in git-annex that runs processes, so it
would work with threading. In the process a place that had forked a process
to feed input to gpg was lost. Fixed it by spawning a thread to feed gpg.
Luckily I have never released a version of git-annex with that bug, but
the many users who are building from the master branch should update.

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Made alerts be displayed while pairing is going on, with buttons to cancel
pairing or respond to a pairing request.