webapp: Better ssh password prompting.

When setting up a remote on a ssh server, prompt for a password inside the
webapp, rather than relying on ssh's own password prompting in the terminal
the webapp was started from, or ssh-askpass.

Avoids double prompting for the ssh password (and triple-prompting on
windows for rsync.net), since the entered password is cached for 10 minutes
and this cached password is reused when setting up the repository, after
the initial probe.

When the user has an existing ssh key set up, they can choose to use it,
rather than entering a password. The webapp used to probe for this case
automatically, so this is a little harder, but it's an advanced user thing.

Note that this commit is known to break enabling existing rsync
repositories. It hs not been tested with gcrypt repositories. It's not been
successfully tested yet on Windows.

This commit was sponsored by Ralph Mayer.

debian/changelog

@@ -6,6 +6,7 @@ git-annex (5.20140422) UNRELEASED; urgency=medium
   * Simplified repository description line format. The remote name,
     if any, is always in square brackets after the description.
   * assistant: Clean up stale tmp files on startup.
+  * webapp: Better ssh password prompting.
 
  -- Joey Hess <joeyh@debian.org>  Fri, 02 May 2014 15:28:53 -0300