Restore edit UI for "Import Only" in Subversion

Summary: Ref T10923. Although I'd ideally like to get rid of this eventually, keep it around for now.

Test Plan:

• Edited value for an SVN repository.
• Observed no panel present for a Git repository.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10923

Differential Revision: https://secure.phabricator.com/D15883