Support configuring export remotes in the webapp

Remember the little-used preferreddir= preferred content
setting and the "public" repository group. The webapp uses
those for IA, which could be replaced with setting up an export
tracking branch.

The UI for S3, WebDAV, directory special remote setup could also have a way
to make it an export, and configure the directory to export. This would
complicate the UI, so needs thought. --[[Joey]]

[[!tag needsthought]]