webapp: Improve handling of remotes whose setup has stalled.

This includes recovery from the ssh-agent problem that led to many reporting
http://git-annex.branchable.com/bugs/Internal_Server_Error:_Unknown_UUID/
(Including fixing up .ssh/config to set IdentitiesOnly.)

Remotes that have no known uuid are now displayed in the webapp as
"unfinished". There's a link to check their status, and if the remote
has been set annex-ignore, a retry button can be used to unset that and
try again to set up the remote.

As this bug has shown, the process of adding a ssh remote has some failure
modes that are not really ideal. It would certianly be better if, when
setting up a ssh remote it would detect if it's failed to get the UUID,
and handle that in the remote setup process, rather than waiting until
later and handling it this way.

However, that's hard to do, particularly for local pairing, since the
PairListener runs as a background thread. The best it could do is pop up an
alert if there's a problem. This solution is not much different.

Also, this solution handles cases where the user has gotten their repo into
a mess manually and let's the assistant help with cleaning it up.

This commit was sponsored by Chia Shee Liang. Thanks!

Assistant/WebApp/routes

@@ -32,6 +32,8 @@
 /config/repository/edit/new/cloud/#UUID EditNewCloudRepositoryR GET POST
 /config/repository/sync/disable/#UUID DisableSyncR GET
 /config/repository/sync/enable/#UUID EnableSyncR GET
+/config/repository/unfinished/check CheckUnfinishedRepositoriesR GET
+/config/repository/unfinished/retry RetryUnfinishedRepositoriesR GET
 
 /config/repository/add/drive AddDriveR GET POST
 /config/repository/add/drive/confirm/#RemovableDrive ConfirmAddDriveR GET