Implement importer "Status" view

* An import event's "View" link loads status info (categorized, paged, and scrollable) for all rows
* From status view, "Back to Uploads" returns to the main "imports" view
* In status rows, highlight cells whose fields had errors or warnings
* For tree imports eliminate "Pending" panel and rename "Tree Watch" as "Warnings"

Refactoring:
* Use server-side templates for everything
* Use `Paginator` instead of homegrown pagination
* Move common row status codes to GenericImportRow
* Extract client-side templates to `clientside_templates.html` (hopefully we'll eliminate most of them)