webapp gpg key generation

Now the webapp can generate a gpg key that is dedicated for use by
git-annex. Since the key is single use, much of the complexity of
generating gpg keys is avoided.

Note that the key has no password, because gpg-agent is not available
everywhere the assistant is installed. This is not a big security problem
because the key is going to live on the same disk as the git annex
repository, so an attacker with access to it can look directly in the
repository to see the same files that get stored in the encrypted
repository on the removable drive.

There is no provision yet for backing up keys.

This commit sponsored by Robert Beaty.

templates/configurators/adddrive/combine.hamlet

@@ -7,7 +7,7 @@
   <p>
     Do you want to combine these files into your repository?
   <p>
-    <a .btn onclick="$('#setupmodal').modal('show');" href="@{FinishAddDriveR (RemovableDriveKey drive Nothing)}">
+    <a .btn onclick="$('#setupmodal').modal('show');" href="@{FinishAddDriveR drive NoRepoKey}">
       <i .icon-resize-small></i> Combine the repositories #
     The combined repositories will sync and share their files.
   <p>