Conflicts: Upload them files if env variable says so

Conflicts: Upload them files if env variable says so

Set OWNCLOUD_UPLOAD_CONFLICT_FILES=1 to trigger this behavior.

Note that this is experimental and unsupported. The real feature is
likely to end up in 2.5.

Uploading conflict files is simply done by removing the pattern from
csync_exclude. The rest here deals with making the conflict notification
ui approximately work.

There are still some concerns about where an uploaded conflict file
appears in the sync protocol and issues list (it should be in both, but
is only in one of them currently!).

See #4557.